54 A \I I'JIIOXUS. 



blast, is called the archenteron or gastrocoel, G : it gives rise to 

 the greater part of the alimentary canal of the larva and adult. 

 The mouth of the cup is called the blastopore; it is at first 

 (Fig. 17) a very large aperture, but in the later stages becomes 

 greatly reduced in size (Figs. 18 and 19). 



Like the blastula, the gastrula is a very widely spread em- 

 bryonic form, occurring not only in Vertebrates, but in a simple or 

 modified condition in certain members of each of the great groups 

 of Invertebrates as well. It has therefore, like the blastula, 

 claims to be regarded as an ancestral form ; claims which are 

 greatly strengthened by the fact that some of the simpler sponges, 

 and some of the Ccelenterates, such as Hydra, are closely com- 

 parable even in their adult condition to gastrulae. 



Fig. 17. Fig. 18. 



Figs. 17 and 18. — Completion of the gastrula of Amphioxus. The embryos 

 are bisected vertically, and one half only of each is represented, x 220. 

 (After Hatschek.) 



Pig. 17. — Completion of the process of invagination, and consequent obliteration of 

 the blastoccel. Pig. 18. — Narrowing of the blastopore, through growth backwards of its 

 dorsal lip. E, epiblast. G-, archenteron or gastrocoel. H, hypoblast. PC, polar meso- 

 blast cell. 



The mechanical causes that lead to invagination, i.e. that 

 actually occasion the change from the blastula to the gastrula 

 condition, are not easy to determine. The epiblast cells appear 

 to take no part in the process, and to undergo no appreciable 

 change or alteration during it ; the active cells in the change are 

 the hypoblast cells. By comparison of Figs. 15, 16, and 17, it 

 will be seen that during invagination there is an increase in the 

 number of the hypoblast cells ; and there is also, which is not so 

 clearly brought out in the figures, an increase in the actual size 

 of the individual cells. This increase in size is perhaps due to 

 the cells absorbing the fluid of the blastoccel ; and this absorption 



