16 THE SUB-KINGDOM CCELENTERATA. 



separates into two halves ; each of tliese, again, 

 into two others, and so on, until, finally, the seg- 

 mented yolk is resolved into an aggregation of 

 minute spherules, forming the so-called * mulberry- 

 mass ' (c). The interior of this mass, in a large 

 number of cases, liquefies, while its outer portion 

 constitutes the * blastoderm,' or ' germinal mem- 

 brane,' from which the several embryonic struc- 

 tures are produced. 



Next, the blastoderm divides into two layers, 

 an outer, or * serous,' and an inner, or ' mucous ' 

 (d). The outer layer more especially contributes 

 to the formation of the organs of animal life, 

 while from the inner layer is fashioned the first 

 rudiment of a large portion of the alimentary 

 canal, and various parts of the body with which 

 it is connected. 



The preceding remarks apply to the ova of most 

 Coelenterate, Molluscous, Annulose, and Vertebrate 

 animals. In all of these, with the exception of 

 the Goelenterata, a further differentiation of the 

 blastoderm would seem to take place. Each of 

 its primary layers divides again into two others. 

 In the outer portion of the serous layer arise 

 the primitive rudiments of the nervous system, 

 while the inner surface of the mucous layer forms 

 the lining of the digestive canal. Between these 

 two structures lies the " membrana intermedia " 

 of embryologists, in the composition of which both 

 serous and mucous layers appear, therefore, to 

 take part. From this membrana intermedia the 

 greater mass of the organic systems which make 

 up the body of the adult animal are subsequently 

 developed. Very soon after the formation of the 

 intermediate layer, a number of important changes 



