192 



ESSENTIALS OF ZOOLOGY 



The dorsal lip during its backward growth continues to yield 

 a continuation of the ectoderm and the endoderm, the former 

 extending the neural plate, and the latter, notochord and 

 coelomic mesoderm. But while it is growing and its products 

 being converted into these structures the rest of the blastopore 

 is enveloping the yolk in ectoderm and endoderm. Not only 



Nucleus and polar bodies 



Ectoderm 



Dorsallip 

 'blastopore 



Neural groove 



Neuropore Forebrain Foregut Enteron Not. Sp. cord 



Primitive 

 streak 



FIG. 97. A succession of diagrammatic longitudinal sections to illustrate 

 the development of Elasmobranchs. 



so, but the margin is also producing a mesoderm which forms 

 the mesenchyme of the yolk between the ectoderm and the 

 endoderm. A continuous mesoderm is thus formed, but it 

 is most pronounced at the dorsal lip and on each side of it. 

 The gastrula therefore passes at once into the postgastrula 

 condition. 



The growth at the posterior end raises the margin of the 

 blastopore from the yolk. A cavity is thereby formed medianly 

 and posteriorly, separating the endoderm from the yolk, and 

 the excavation extends into the primitive gastral region, to 

 the anterior end of the embryo. This end of the embryo at 



