EMBR YONIC DE VEL OPMENT. 



I I I 



by the continued deepening of the archenteric cavity 

 (Fig. 60). 



It is now a well-established fact that all multicellular 

 animals (Metazoa) pass through a gastrula-stage in the 

 course of their development, although the form of the 

 gastrula is often extremely modified and difficult to recog- 

 nise. The significance of this 

 fact, as was long since pointed 

 out by Huxley, Haeckel, Lan- 

 kester, and others, is very 

 great when it is remembered 

 that the embryonic character- 

 istics of the gastrula are 

 essentially identical with the 

 permanent features of the 

 organisation of the Ccelen- 

 terata (Hydra, etc.). 



Returning to the gastrula begins to rotate within tlie follide. 

 ;- . , . . ,, (After HaTSCHEK.) 



of Amphioxus, m the course 



of the further differentiation which goes hand in hand 

 with the progressi\'e growth and development, we shall 

 find that the primitive ectoderm gives rise to (i) the 

 central nervous system and (2) the definitive ectoderm ; the 

 primitive endoderm gives rise to (i) the mesoderm, which 

 is usually regarded as a third or intermediate germ-layer ; 

 (2) the notocliord ; and (3) the definitive endoderm, which 

 forms the lining mucous epithelium of the alimentary 

 canal ; finally, the primitive gastral cavity or archenteron 

 will become subdivided into (i) the body-cavity or eadom, 

 and (2) the definitive gut or alimentary canal. 



The embryo shown in optical section in Fig. 60 repre- 

 sents the stage reached at midnight of the first night of 

 development. It will be noticed that one side is convex, 



Fig. 60. — Optical longitudinal sec- 

 tion of later gastrula. Cilia (flagella) 

 have been protuded from the ectoderm 

 cells, and the embryo at this stage 



