EMBRYOLOGY 



221 



of cells, the original animal half, forms the ectoderm; the inner layer, 

 the vegetative half of the blastula, is now the endoderm. Within the 

 layers of cells is a new cavity, the archenteron, which is connected with 

 the outside by a small opening, the blastopore. 



Gastrulation in Telolecithal Embryos. — Blastulas containing much 

 yolk are invaginated in a somewhat different manner. The lamprey 

 (Fig. 176) illustrates the difference very simply. Owing probably to the 

 difficulty of turning in so thick a layer of yolk-laden cells as is present 

 in the vegetative half of the embryo, invagination starts at one side, as in 

 Fig. 176, A, approximately half way between the animal and vegetative 

 poles. The inward movement of these cells proceeds until the blasto- 

 coele is eHminated, as in homolecithal embryos. Ectoderm and endo- 

 derm are thus produced, and an archenteron and blastopore, as illustrated 

 in the figure; but the archenteron is a very much flattened cavity. The 

 endoderm is in part a comparatively thin 

 layer, but in part (the original vegetative 

 part of the egg) a very thick layer. 



In blastulas of this type gastrulation is in 

 part accomplished by two processes that are 

 not invagination. The thin layer of cells 

 above The'HIastoccele, which later becomes in 

 part ectoderm, at first includes only a fraction 

 of the surface of the blastula (see Fig. 176, A). 

 The ectoderm is extended, during gastrula- 

 tion, by a process known as delamination. 

 As shown in D, a layer of cells is split off from 

 the outer part of the original vegetative part of 

 the embryo, thus forming a continuation of 

 the ectoderm. 



The other process leading to the forma- 

 tion of the gastrula is one of over-growth. At the lip of the blastopore, 

 where the ectoderm and the thinner part of the endoderm meet, these 

 two layers undergo an active growth, thus creeping down over the vege- 

 tative pole of the embryo, as shown at the left in Fig. 176, C and D. 



The end result is a two-layered embryo, the gastrula; but that result 

 is reached by over-growth and delamination, as well as by invagination. 



Gastrulation in the telolecithal embryos of reptiles and birds and in 

 the embryos of mammals is so much modified that these forms cannot be 

 here included. Likewise, the early foldings of the layers of cells in de- 

 veloping centrolecithal embryos must be omitted. 



Mesoderm Formation. — At the end of gastrulation two layers of cells, 

 ectoderm and endoderm, are present. In all but two groups of metazoa 

 low in the animal scale, a third layer is soon formed between these two. 

 This layei is the mesoderm. 



Fig. 175. — Gastrulation in 

 the holothurian Synapta. ar, 

 archenteron; b, blastoccele. 

 {From drawing by P. O. Okkel- 

 benj.) 



