In the chick embryo, the tail bud reaches its maximum length at about stage 22 of Hamburger and Hamilton, after which it starts to regress. By this stage the neural tube and notochord extend right to the tip of the tail, but the somites do not do so, the terminal tail bud mesoderm never becoming segmented. The investigation is concerned with analysing why this mesoderm fails to segment. When tail buds were explanted to the chorio-allantoic membrane, they continued to form somites only until the "correct" number had segmented, i.e., the tail bud formed no more somites when isolated from the embryo than it would have formed if undisturbed. Morphological studies suggest that in the normal embryo massive cell death overtakes the tail bud mesoderm before it can segment. It is suggested therefore that cell death may be a contributory factor in preventing segmentation.
The morphology of the Malpighian tubules and associated structures in the larva and adult of Philosamia ricini Hutt has been described and an account has been given of the embryonic development of, and post‐embryonic changes in, these structures. The proctodaeum is believed to be entirelyectodermal and so are the Malpighian tubules and the vesicles derived from it. The development of cryptonephric condition and its disappearance duringmetamorphosis have been worked out in detail. The pattern of growth and differentation of the Malpighian tubules in the larval instars has been studied.
Small graft pieces (size: 0.15xO.2mm to 0.1 x0.4mm and 0.25x0.25 mm) were isolated from the prospective prosencephalic ectoderm of the definitive primitive streak (st. 4) and head process (st. 5) chick blastoderms, and cultured in vivo intracoelomically. In all 1437 graft pieces isolated from 448 donor blastoderms were transplanted into the coelom of 941 host embryos; 216 hosts died before reaching 12 days. The 725 surviving hosts, which carried 1241 implants, yielded 304 analysable grafts. The very small size of the isolates affected the recovery rate. The histological analysis of the recovered grafts reveals that the frequency of neuralization was higher in the grafts isolated from the central region of the prospective prosencephalic ectoderm than in those from its peripheral region. A centrifugal extension of Merentiation tendencies of prosencephalic structures was observed from st. 4 to st. 5. At st. 5 frequency of telencephalic structures was high in the grafts of the anterior region, whereas that of diencephalic structures was high in those of the more posterior region. Eye structures (retina as well as tapetum) differentiated in an area measuring ca. 0 . 4~0 . 1 mm extending transversely about 0.1 mm anterior to the head process at st. 5.Structural elements of the lens were not observed in any graft.Attempts to understand the process of neural induction in the early chick embryo date back to the work of WADDINGTON (1 , 2), who first proposed the role of the head process mesoderm as neural inductor. HARA (3), based on his sandwich experiments, demonstrated that prechordal mesoderm induces prosencephalic differentiation tendencies in the competent ectoderm; anterior part of the head process induces mes-/rhombencephalic differentiation tendencies ; while its posterior part induces rhombencephalic and spinal cord differentiation tendencies. RAO (4) analysed the pattern of appearance and extension of regional differentiation tendencies in the prospective neural ectoderm. However, his isolates were large in size (e.g. his median piece covered 3/4 of the width of the prospective neural ectoderm) and he did not study differentiation tendencies of various,components of the brain in more detail. In the present work, differentiation tendencies of much smaller pieces of prospective prosencephalic ectoderm have been tested; and an attempt has been made to analyse the pattern of neural differentiation in this region. Special attention has been paid to the localization of the eye potency field, and results are discussed in comparison with the earlier findings (5, 6, 7), which were based on the differentiation of test pieces comprising all three germ layers.
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