FATE OF BLASTOPORE, ETC., IX CHELOXfA. 23 



the Blastopore. Almost simultaneously with this uct of invagination, 

 or more probably ecen before it^ the epiblast extends back to 

 the anterior lip of the Ijlastopore, so that from very early stages there 

 is in front of the blastopore no area free from the epiblast. The cells 

 of the primitive plate now begin to spread themselves over the ventral 

 surface of the embryonic shield, mainly in the anterior direction, and 

 jtavi jmssu with it, the archenteron extends itself forwards (Woodcut 

 IV. C). The roof of the archenteron, which becomes continuous with 

 the epiblast of the embryonic shield at the anterior lijj of the blasto- 

 pore, assumes a columnar Jirrangement, the process beginning at the 

 posterior end and proceeding gradually forwards. Out of the median 

 part of it is established the cZ/ortk-hypoblast, and from a certain stretch 

 of columnar epithelium on each side of it is developed the gastral 

 mesoblast (See C out rib. III. for details). The floor of the archenteron 

 is divided into two parts : — (c/) the posterior which proliferates the 

 peristomal mesoblast, and (Jj) the anterior which, losing definiteness, 

 is finally absorbed together with the whole thickness of the cell- 

 network placed beneath it, thus putting the archenteron in com- 

 munication with the large subgerminal fluid-cavity which has been 

 developing in the yolk beneath the embryo (Woodcut IV. D). After 

 this breaking through on the part of the archenteron, the cells derived 

 from the primitive knob spread themselves gradually over the entire 

 ventral face of the embryonic shield in the form of a trabecular network 

 (Woodcut IV. E). This, however, soon becomes changed into a 

 compact sheet of cells, tlie process beginning at the part that 

 corresponds to the original roof of the invagination cavity and 

 gradually spreading centrifugally. A median transverse fold arises 

 soon at some distance behind the anterior edge of the embryonic 

 shield (Woodcut IV. F, Hd. fold) and marks the head of the 

 embryo, and the anterior end of the archenteron. The whole embryo 



