The Microscope. 



83 



germ layers, the epiblast and hypoblast. The latter, however, 

 is becoming folded, and forms two longitudinal pouches either 

 side of the median line. With growth these become constricted 

 off, their cavity becomes separate from that of the archenteron, 

 and finally the walls themselves become separated. With this 



Fig. 6. Section of embryo of Amphioxus. a, archenteron ; c, ccelomaiic 

 pouches; ch, notochord; m 1 , somatoplure; m2, splanchuoplure; n, neural cord. 



Fig. 7. Egg of Newt, e, epiblast; h, hypoblast; mi, somatoplure; m2, 

 splanchnoplure; o, blastopore. 



separation the differentiation of the mesoblast is accomplished 

 and the cavity of the mesoblastic pouches becomes the body 

 cavity, or the ccelom. 



This description applies, mutatis mutandis, to the origin of 

 the mesoblast in various other forms — Sagitta, Peripatus ca- 

 pensis,* Balanoglossus, and several other forms. With an in- 

 crease in food yolk the process becomes considerably modified, 

 the pouches being brought nearer the mouth of the gastrula, 

 until at last, as in the frog and newt, the mesoblast arises, not 

 as pouches, but as solid bands, from the lips of the blastopore. 

 All through our investigations we will constantly be brought 

 into contact with food yolk and the influence it has on the 

 growing germ. Indeed it is hardly possible to estimate its im- 

 portance too highly. When abundant, the embryo can stay a 

 long time in the egg, and there undergo most of its metamor- 

 phoses; when scanty, it is early compelled (except in the case 

 of mammals) to leave the protective envelopes at an early 

 stage of its development, and to undergo its changes in a state 



* According to Kennel, another process occurs in two South American species 

 of Peripatus. 



