STAGE A. 11 



was placed, the endoderm cells may be seen for a short time 

 projecting (figs. 17 and 18). 



The embryo has thus acquired a spherical form, and consists 

 of a solid gastrula, the small uncovered spot of endoderm con- 

 stituting the blastopore. A cavity next appears in the centre 

 of the endoderm cells, so as to open to the exterior through 

 the blastopore (figs, 19 and 21). 



We have thus arrived at the stage of a typical gastrula formed 

 of two layers of cells, which are continuous with one another at 

 the blastopore and enclose a central cavity. It may be at 

 once stated that the blastopore, which is on the ventral surface 

 of the embryo — on the surface opposite to that on which the 

 ectoderm cap was placed — persists and gives rise to the mouth 

 and anus^ of the adult, and that the cavity of the gastrula 

 becomes the mesenteron. 



The General Features or the Embryo at Successive 



Stages. 



The segmentation is then apparently complete but unequal; 

 the large cells giving rise to the endoderm, and the small cells 

 to the ectoderm. The gastrula arises by a modified process of 

 epibole. The fully-developed gastrula is shown in figs. 19 and 

 21. The embryo has already become slightly oval, and the 

 blastopore now begins to elongate in the direction of the long 

 axis. 



Stage A (fig. 22). — An opacity appears at the hind end of the 

 blastopore. This opacity is the primitive streak. It appears 

 to be due to the active proliferation of some cells, which cannot 

 be definitely assigned either to the ectoderm or the endoderm, 

 at the hind end of the blastopore. This stage, which has 



1 These were called in Balfour's memoir ('Q. J. M. S.,' 1883), and perhaps 

 more correctly, the embryonic mouth and anus, — more correctly because 

 they come in the adult to lie internally, in consequence of the ingrowth of 

 ectoderm at the two ends of the alimentary canal to form the stomodseum and 

 proctodseum. They constitute in the adult the openings between the mesen- 

 teron and the stomodseum and proctodajum respectively. It must, however, 

 be borne in mind that they never become closed. 



