54 



INVERTEBRATE MORPHOLOGY. 



Fig. 35.— Diagbam of a Gastkdla. 



blastula by the pushing in or invagination of the cells of one 

 pole (the posterior in free-swimming blastulas) into the blas- 

 tocoel, which thus becomes more or less perfectly obliterated. 



The cavity lined by the iuvagi- 

 nated cells is the primitive di- 

 gestive tract or archenteron, its 

 opening to the exterior being 

 the gastrula-mouth or blastopore. 

 The gastrula is a two-layered 

 organism or is diploblastic, and 

 the cell-layers of which it is 

 composed are the primitive germ- 

 layers. The outer layer in the 

 higher Metazoa gives rise to the 

 integument, nervous system, and 

 sense-organs of the adiilt and 

 is known as the ectoderm, while 

 the inner one, from which the digestive tract and its glands, 

 such as the liver, will develop, is termed the endoderm. 



Just as the presence of yolk in the ovum may modify the 

 segmentation, so too it may produce decided modifications in 

 the formation of the gastrula. The method just described, 

 which occurs in embryos containing little food-yolk, is distin- 

 guished as embolic from the epibolic method occurring in telo- 

 lecithal ova which undergo a markedly irregular segmentation. 

 In such ova, as has been stated, one pole is occupied by inert 

 yolk-laden spherules, while at the other are almost yolkless 

 active cells. These latter divide rapidly and extend as a cap 

 over the yolk-laden cells and finally completely enclose them. 

 The result is practically the same as in the embolic method, 

 the yolk-laden endoderm cells being enclosed within the yolk- 

 less ectoderm. 



Among the lower Metazoa especially, another method oc- 

 curs by which the diploblastic embryo is formed. Instead of 

 certain cells invaginating, each cell of the blastula divides in 

 a plane parallel to the surface of the organism, one of the two 

 cells thus produced becoming ectoderm, while the other is a 

 portion of the endoderm. A diploblastic closed sac thus re- 

 sults, the blastopore appearing later and placing the archen- 



