Previous studies of the spontaneous cyclic motility of the chick embryo were extended to incubation day 17. The activity and inactivity phases of the cycle were recorded on a polygraph, for a 15-minute observation period. The percentage of time spent in activity rises steadily from less than 10% at the beginning of motility at three and one-half days to 80% at day 13. This peak is maintained up to day 17; subsequently, motility declines. The mean duration of activity phases increases and that of the inactivity phases decreases until the high plateau is reached, though both values change independently of each other. I n order to study the effect of the brain on spontaneous motility, spinal embryos were obtained by extirpation of part of the cervical cord in two-day embryos. The spinal embryos retain their capacity for cyclic motility. However, the activity is reduced by approximately 1 6 2 0 % at all stages between the eighth and the seventeenth day. The brain influence asserts itself in a lengthening of the duration of the activity phases and in a shortening of the inactivity phases. A n inhibitory effect was not observed at any stage.

The following experiment was designed to test the role of sensory input in the motility of the chick embryo: Complete deafferentation of the leg level was accomplished i n 2-day embryos by extirpation of the dorsal half of the lumbar spinal cord including the precursor cells of spinal ganglia; and simultaneous extirpation of the entire spinal cord i n the thoracic level to exclude sensory input from more rostra1 levels. In control embryos, only the thoracic gap was made. The embryos were reared to eight and one-half, 11, 13, 15 and 17 days, respectively, and manual recordings were made of the leg motility before the embryos were sacrificed. It had been reported by us that the motility of normal chick embryos is periodic, activity phases alternating with inactivity phases. Both the overall activity, i.e., percent of activity during a standard 15-minute observation period, and the durations of activity and inactivity phases were found to be the same in experimental embryos and in control embryos with thoracic gaps, a t least up to and including 15-day embryos. Histological checks showed the complete absence of lumbar spinal ganglia i n the majority of embryos and, the presence of a few small thoracic ganglia in some cases. All embryos with lumbar ganglia were discarded. The motor roots formed a normal plexus, and the pure motor nerves emerging from it showed a normal peripheral distribution and muscle innervation. It is concluded that the leg motility results from autonomous discharges of the lateral motor or internuncial neurons, and that sensory input is not necessary for the triggering, the maintenance or the periodicity of the leg motility. A sharp decline of motility was observed i n 17-day embryos. A study of the histological condition of the residual spinal cord showed a numerical depletion and partial degeneration of the lateral motor column. This deterioration is considered to be responsible for the functional impairments.The implication of these results for the widespread notion that "self-stimulation" and similar "experiences" during embryonic development play a n important role in the molding of behavior is discussed. Dedicuted t o Dr. J . Holtfreter, o n the occasion o f his 65th birthdayIn previous publications (Hamburger, '63; Hamburger and Balaban, '63; Hamburger, '64; Hamburger, Balaban, Oppenheim and Wenger, '65) we have characterized the motility of the chick embryo as follows : ( 1 ) It begins at three and one-half days with a slight turning of the head at long intervals of approximately one minute and builds up to a peak of activity that is reached at 13 days, when the embryo is in motion for 80% of the time. ( 2 ) Motility is periodic; activity phases alternating with inactivity phases. ( 3 ) The movements are random movements of different parts of the body, with no obvious coordination of the parts. It was found furthermore that no triggers from higher centers are required for body movements. Following early embryonic isolation of the spinal cord from the brain, periodic body and li...

Counts have been made of the number of cells in the trochlear nucleus in a series of 22 normal chicks between the ninth day of incubation and the seventy-fifth day after hatching, and in seven animals in which the optic vesicle and adjoining mesoderm were removed during the second day of incubation. Between the ninth and seventeenth days of incubation the number of cells in the normal trochlear nucleus declines from just under 1400 to a little under 700, indicating a cell loss of approximately 50% over this eight day interval. From the seventeenth day onwards there is a further slight fall to between 500 and 600 cells and this number is maintained into the third month after hatching. Following excision of the optic vesicle there is an earlier, and more severe, loss of neuxons in the trochlear center, the number of cells being reduced to just under 1000 at the eleventh day of incubation and falling to just over 300 by the sixth day after hatching. Despite this marked cell loss, at no stage is there any gross evidence of neuronal disintegration or phagocytosis. The possible factors responsible for the normally occurring cell loss and for the hypoplasia in the trochlear nucleus after removal of the optic vesicle are discussed.In the 60 years since Collin ('06) first reported the presence of degenerating cells in the ventral horn of the chick spinal cord between the third and sixth days of incubation, a good deal of evidence has accumulated to show that cellular degeneration may be both of widespread occurrence and of considerable morphogenetic significance in the developing nervous system. Unfortunately, this phenomenon appears to have received rather less attention than it deserves, no doubt because most of the available evidence is in the form of incidental references either to the occurrence of cell loss or to the presence of degenerating neurons in different parts of the central and peripheral nervous systems. However, a few authors have made this the subject of special study (e.g., Ernst, -26; Glucksmann, '30; Hamburger and LeviMontalcini, '49; Hughes, '61 ; Prestige, '65; Bodian, '66) and it has also been considered at some length in three recent reviews (see Glucksmann, '51 ; Hess, '57; and K5l-len, '65). Our attention was drawn to this phenomenon during the course of an experimental study of the development of the avian visual system (Cowan and Wenger, '67) when we observed what ap-J. EXP. ZOOL., 164: 267-280. peared to be a slow, but progressive loss of neurons in the nuclei of origin of the third, fourth and sixth cranial nerves. Preliminary counts of the number of neurons in the trochlear nucleus indicated that there may be a loss of as many as 50% of the cells in this center between the ninth and twentieth days of incubation. In view of the magnitude of this cell loss, and especially since no comparable change was observed in an earlier experimental study of the trochlear nucleus (Dunnebacke, '53), it was considered worthwhile to carry out a more complete study of the number of cells in this n...

The number of mitotic figures in the optic tectum of the chick increases from just over 20,000 on the fourth day of incubation (stages 25 and 26) to a peak of approximately 45,000 on the fifth and sixth days (stages 28 and 29). Thereafter, the number declines sharply to about 10,000 and 2,000 on the eighth and ninth days respectively and to as few as 100 and 18 on the eleventh and twelfth days. The dividing cells are not uniformly distributed throughout the tectum but show a clear rostral to caudal gradient such that at every stage the number of mitoses in the caudal half of the tectum exceeds that in its rostral half. In addition the number of mitotic figures in the ventrolateral half of the tectum exceeds that in its dorsomedial half between the fourth and seventh days of incubation, but this pattern is reversed during the last few days of mitotic activity.The process of cell proliferation in the optic tectum appears to be completely independent of the developing eye since there is no significant change in either the total number of mitotic figures in the tectum, or in their spatial distribution, after complete removal of the optic vesicle towards the end of the second day of incubation.Interest in mitotic activity in the developing nervous system has been focused principally upon two problems : the mechanism and timing of cellular division within the neural epithelium, and the role of peripheral factors in regulating cell proliferation in the neural tube and its derivatives. The extensive literature dealing with the first of these problems has recently been reviewed in detail by Watterson ('65) and need not be considered here, although it may be pointed out that there is still no generally accepted explanation for the inward movement of the dividing nuclei towards the luminal surface of the neural epithelium, or for the subsequent separation and outward migration of the daughter cells (see Martin, '67). Rather less attention has been paid to the significance of extrinsic factors in the control of mitotic activity in the nervous system. The findings in a number of studies have been quoted in support of the hypothesis that there is a peripheral regulative effect upon cell proliferation but, as Hamburger ('48) has stressed, much of this evidence is rather indirect and inconclusive. For example, in many experimental studies care has not been taken to distinguish clearly J. EXP. ZOOL., 169: 71-92. between the effects of peripheral changes upon mitotic activity per se, and later effects such as disturbances in neuroblast differentiation and growth, or even degenerative changes in differentiated cells. The evidence which is available from studies based upon actual mitotic counts (rather than total cell counts or volumetric measurements) is somewhat conflicting and at present it is by no means clear to what extent the apparent discrepancies reflect real differences between the central and peripheral nervous systems or between different groups of vertebrates.In one of the earliest quantitative studies of ...