ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 339 



Development of Chelonia.* — A. Bracket follows up his earlier work 

 on development of the head in Amphibians, with an account of acro- 

 genesis, cephalogenesis, and cormogenesis in Chrysemijs marginata. 

 His general conclusions, which differ in many respects from those of 

 other investigators, are as follows : The cephalic prolongation of the 

 primitive plate, after it has insinuated itself between the embryonic 

 ectoblast and the vitelline endoblast, and after the blastoporal canal has 

 opened into the sub-germinal cavity, constitutes an extended cephalic 

 endoblast circumscribed laterally and anteriorly by the vitelline endoblast 

 arising from the gastrular cleavage. 



The ectoblastic plate is at first much more extended than the 

 cephalic endoblast, but the ectoblast soon contracts ; it thickens, glides 

 over the surface of the vitelline endoblast, and re-forms exclusivelv above 

 the cephalic endoblast and at the periphery of the embryonic shield 

 where the protochordal or prechordal plate persists. The cephalic 

 endoblast, which is vacuolar, and at first considerable in extent, becomes 

 reduced and more compact as a result of a contraction analogous to, but 

 less marked than that of the ectoblast, and of the proliferation of its 

 cellular elements. It thus takes on the aspect of a cylindrical epithelium 

 in which the nuclei are stratified in two or three layers. When this 

 concentration has taken place the blastoporal region contracts in the 

 same directions as the rest of the blastoderm, and the dorsal lip recedes 

 slightly. A zone is thus formed between the cephalic endoblast and the 

 blastopore and it immediately proliferates intensely, behaving as a zone 

 of growth which will elongate the part of the body in front of it. 



As soon as the zone of growth has become active, it is possible to 

 distinguish in the endoblast of the embryo of Chrysemys three zones : 

 {(l) an anterior narrow band of vitelline endoblast corresponding topo- 

 graphically to Hubrecht's " protochordal plate ; (b) the cephalic endo- 

 blast which arises from the cephalic prolongation of the primitive plate ; 

 and (c) the internal layer of the zone of growth which is contiguous to 

 the blastopore and surrounds what is left of the blastoporal canal. It 

 elongates rapidly and is from the beginning a very active mesoblasto- 

 genic centre. Its formation is secondary and follows that of the 

 cephalic endoblast. 



When the anterior extremity of the body is defined, the protochordal 

 plate is in part left out of the embryo ; the remainder invests the lower 

 part of the anterior cul-de-sac of the alimentary canal, and forms a part 

 of its ventral wall. It does not share in the formation of the notochord, 

 but it may play a role in the formation of the premandibular mesoblast, 

 and thus take part only in acrogenesis. 



In the region of the cephalic endoblast the two primary layers 

 develop in a very characteristic manner. The ectoblast differentiates 

 into medullary plate and epiblast. But the latter is not a mere investing 

 layer, for from the very "first it possesses neural characters, evidently 

 associated with the sensory elements it is called upon to form. The 

 endoblast subdivides into three zones — a median, which will become the 

 notochord, and two lateral zones ; the internal part of these latter gives 

 rise by proliferation to all the mesoblast of the head (except the pre- 



* Arch. Biol., xxix. (1914) pp. 500-77 (3 pis.). 



