ASCIDIAOEA — FORMATION OF THE GERM-LAYEKs. 341 



sixty-four-cell stage. For further details as to the cleavage we must 

 refer the reader to the works of Seeliger (No. 50), van Beneden 

 and Juein(N'o. 8), (hahrv (Nos. 12 and 13) [and especially Castle 

 (No. II.)]. 



Even at the four-celled stage a cleavage-cavity is found open at 

 both the animal and vegetative poles (Fig. 150). This cavity, at the 

 sixteen-celled stage, appears to he closed on all sides. In later stages 

 it disappears (Fig. 151 A,f) in consequence of a flattening of the 

 embryo, which begins at the poles and which is specially marked 

 in the entodermal half of the hody ; this flattening precedes the 

 invagination of the entodermal cell-layer which results in the 

 gastrula-stage. 



C. Formation of the Germ-layers. Appearance of the Medullary 



Tube and the Notochord. 



Through the changes just described, the embryo passes from the 

 blastula-stage into a stage which we may, with Butschli, call the 

 placula (Fig. 151 A). The lens-shaped embryo is now composed of 

 two layer's, an entodermal layer (en) consisting of large high cells 

 and a small-celled ectodermal layer (ec) which already covers the 

 former like a cap.* In a fissure (/) between these two layers, we 

 recognise the remains of the much compressed cleavage-cavity. The 

 gastrulation which now takes place (Figs. 151 B, and 152) is due 

 essentially to the curvature of the bilaminar embryo, the flattened 

 area of entoderm-cells becoming invaginated in this process, while 

 the ectodermal layer continues to spread out over the surface of the 



orientation of the Ascidian egg. Castle concludes that Seeliger determined 

 the dorsal and ventral sides of the egg correctly, but reversed the anterior and 

 posterior ends in all his figures of the early stages, van Beneden and Julin 

 were correct in their determination of the anterior and posterior ends, but 

 reversed the dorsal and ventral surfaces in all stages prior to the forty-four- 

 celled stage. In consequence, the latter authors state that the four small cells 

 of the eight-celled stage give rise to ectoderm only, while the four larger cells 

 produce both entoderm and ectoderm ; whereas, as a matter of fact, neither 

 group produces ectoderm exclusively. It is the four larger, not the four smaller 

 cells, which give rise to the greater portion, perhaps the whole, of the ectoderm. 

 The vegetative pole, which is marked by the polar bodies and the four smaller 

 blastomeres, is dorsal, while the animal pole with the four larger cells is 

 ventral. The future mesoderm arises from both primary layers. Castle's 

 work is so complete, and he traces the cell-lineage in such detail, that his 

 interpretation must, we think, be accepted in preference to the above. — Ed.] 

 * [The large cells at this .stage would not, according to Castle, correspond 

 with the large cells of the eight- and sixteen-celled stage ; the latter by their 

 more rapid division have become gradually smaller, whereas the originally 

 smaller cells, by their slower division, have not decreased in size to the same 

 extent and are now the larger of the two. — Ed.] 



