THIRTY-EIGHT FIGlJRES INTRODUCTIONThat the mesodermic somites exert a morphogcnetic role upon the development and segmentation of the spinal ganglia is a fact that has been supported by a number of experimental studies (Lehmann, '27 ; Detwiler, '32, '33 a, '33 b). Working up011 tlic embryos of Plenrodeles shortly after the closing of the neural folds, Lehmann excised the somites as well as grafted a region of the spinal cord lateral to thc host's somites. From his findings Lehmann drew essentially the following coriclusioris : 1) that in the complete absence of the somites of a definite i*egion, the developmcnt of the respective spinal ganglia is entirely suppressed (p. 102) ; 2) "that the segmental arrangement of the spinal ganglia o~w r s only wlieii therc is a degree of normal arrangement of the mesodermal structiires, and that in almost all cases disturbances in the mesoderm are accompanied by an abnormal develo1)-meiit of tlie ganglion cells"; 3) that the normal location of the sensory and motor roots is subservient to a normal arrangement of the mesoderm, and that disturbances in the latter bring about alteration in the location of the out-growing fiber bundles; 4) that it is the presence of thc mesial surface of the somites mhich brings about differentiation and segmen- iation of the ganglia. The lateral surface of tlic somite lacks this formative quality, according to Lehmann, for when the cord was grafted lateral to the sornites, segmentation in the ganglia failed and little or no differentiation of cells occurred.Similar. experiments by tlie author upon the embryos of Amblystoma ('32) have yielded a number of cases in which discrete spinal ganglia developcd and differentiated in tlie complete absence of developing muscle and cartilage-a condition which, according to Lehmann, did not exist in Pleurodelcs. Moreover, a number of eases were obtained in which spinal ganglia developed along the lateral surface of the host 's somites when tlie spinal cord WRS grafted to this region. That tlie lateral surface of the somite inhibits ganglion cell differentiation, as coiicluded by Lehmann, could not be substantiated in my experiments.From the results so far at hand, it would liardly seem justifiable for us to conclude that no differentiation of spinal ganglion cells can take place in the absence of the mesial face of the somite or of the axial skeleton regardless of the morpliogenctic influence these tissues may exert under normal conditions of development. It would appear that the crest cells possess a certain amoiint of self-differentiating capacity in the same sense that Harrison ('10) foirnd the embryonic neuroblasts to develop in vitro. It may be that tissues other than developing myotome or sclerotome maS influence differentiation, for in several of my cases ganglia developed when adjacent 1 o glandular tissue (pronephros) and the notochord.The most conviiicing evideiice supporting the view that spinal ganglia can develop and diff ercntiate in the absence of muscle and cartilage was obtained from those case...

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Estudios sobre la retina.La estructura de la retina de Slligator mississipiensis y sus cambios fotomechnicos.El ojo de Alligator posee un tapetum retinal bien desarrollado, formado por la inclusi6n de guanina en las c6lulas epiteliales de las porciones dorsal y posterior, a una distancia de 1.5 mni. de la entrada del nervio 6ptico. El pecten consiste en una especie de copa pigmentada ligeramente elevada, que cubre la entrada del nervio 6ptico. E n toda la retina se encuentran conos y bastones, pero la proporcih de anibos es diferente en distintas regiones. Los bastones son todos del mismo tipo, 10s conos de dos tipos: grueso y delgado. Los primeros son mhs numerosos, presenthdose especialmente en las regiones posterior y ventral de la retina. Los conos del segundo tip0 se encuentran solamente en la porcibn ventral y no son numerosos. T a m b i h existen conos dobles. Ninguno de 10s conos y bastones contiene gotas de grasa.Los nlicleos de 10s bastones son de forma oval y la mayor parte de ellos se proyectan a travks de la membrana limitante externa en una extensi6n variable. Los nGcleos de 10s conos son piriformes y ocupan un nivel m8s profundo que el de 10s nficleos de 10s bastones, formando una segunda fila. Los bastones presentan un cambio de longitud media de unas 4 micras, y son m8s largos a la luz. Los conos sencillos presentan un cambio medio de 2.1 micras y son m8s cortos a la luz. Los mienibros mas pequefios de 10s conos dobles presentan un cambio medio de longitud de 3.5 micras, y 10s mayores de 2.7 micras. La emigracih del pigmento es ligera, y su media es 1.6 micras, per0 cuando se combina con el cainbio de longitud de las c6lulas visuales produce una emigraci6n efecti va igual a su sunia. El trabajo termina con consideraciones te6ricas sobre 10s cambios fotomec&nicos y la teoria de la duplicidad bajo un punto de vista comparado.

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I n a study of the influence of function upon the differentiation of skeletal muscle, Harrison ('04) narcotized frog embryos (R. virescens and R. palustris) with chloretone during the period when the swimming reflexes develop. Embryos were placed in chloretone solutions of various strengths just prior to the onset of initial reactions and were kept in a narcotized state for a period of seven days. I n one experiment listed (Harrison, '04, p. 210, exp. 6) embryos of both species were narcotized for seven days iii a 0.03 per cent solution and then placed in tap-water. After eleven minutes in water, the virescens embryo reacted with a quiver or jerk, and in seventeen minutes it was able to swim across the dish.The palustris embryos required a longer period for recovery. At the end of fifty-one minutes they reacted with a jerk, and in one hour and seven minutes they were able to swim across the dish.I n another more striking case, an embryo of R. virescens reared in a 0.02 per cent solution was able to swim several strokes after five minutes in water, and showed good coordinated swimming movements in seventeen minutes. I n all these experiments, the palustris embryos recovered more slowly than the virescens embryos.Harrison's observations are of interest in showing that, iii spite of the total absence of response to stimulation during the developing reaction period, a normal swimming reflex pattern developed. It was also observed that, under complete 2 i 9

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Estudios sobre la retina.La estructura de la retina de Phrynosoma cornutum.La estructiira general de la retina de Phrynosoma es muy semejante a la de la retina de Chameleon. Existe un Brea central muy desarrollada en forma de una convexidad circular situada encima del punto de entrada del nervio 6ptico, conuna fovea que alcanza el mliximum de desarrollo en su'centro. Un pecten c6nico muy vascularizado se proyecta dorso-anteriormente hasta cerca de un milimetro en la camara posterior del ojo. La retina posee solamente conos, asemajhndose bajo este punto de vista a la retina de otros saurios diurnos. Los conos exhiben una variaci6n considerable en tamafio, forma y estructura. Los conos de la fovea estBn muy atenuados y son cilindricos, diferencihndose por estos caracteres de 10s de forma chnica, situados en la regi6n extra-foveal. La presencia de conos dobles no ha podido demostrarse. Bajo condiciones ordinarias de iluminacih el pigmento se extiende sobre las cklulas visuales, llegando casi hasta las paraboloides, except0 en la regi6n de la fovea, en la cual solo 10s segmentos externos estBn recubiertos por el.

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