DEVELOPMENT OF THE ORGANS. 85 



ing of a single layer of cells. This stage is known as the hhutula 

 (or hlastosphere) (J., B^ Fig. 35). 



The formation of the germ-layers is one of the most im- 

 portant and signillcant processes in the whole course of devehjp- 

 ment. Germ-layers like those of Lmnhrlrus^ and callt-d hy 

 the same names, are found in the embryos of all higher ani- 

 mals; and it will hereafter appear that this fact has a })n (found 

 meaning. 



Development of the Organs. (Organogeny.) The embry(j gradu- 

 ally increases in size and at the same time elongates. As it 

 lengthens, the blastopore (in this case the mouth) remains at one 

 end, wliicli is therefore to be regarded as anterior, and the 

 elongation is backwards. The cells of all three germ-layers 

 continually increase in number by division, new matter and 

 energy being supplied from the food, which is swalhjwed by the 

 embryo in such quantities as to swell up the body like a bladdrr. 

 The archenteron enlarges until it comes into contact with the 

 ectoblast and the segmentation-cavity is obliterated. 



The two primary mesoblastic cells are carried backwards, 

 and always remain at the extreme posterior end (???, Fig. .ST)). 

 The mesoblast is in the form of tw^o bands Ivino: on either side 

 of the archenteron, and extending forwards from the primary 

 mesoblastic cells. 



This is clearly seen in a cross-section of the embryo, as in 

 Fig. 36, B^ C. The mesoblastic bands are at lirst solid, but 

 after a time a series of paired cavities appears in them, con- 

 tinually increasing in number by the formation of new cavities 

 near the hinder end of the bands as they increase in length. \ 

 cross-section passing through one pair of these cavities is shown 

 at B^ Fig. 35. As the bands lengthen they also extend up- 

 wards and downwards (6^, Fig. 35), until finally they meet al)ove 

 and below the archenteron. The cavities at the sanio time 

 continue to increase in size, and finally meet above and below 

 the archenteron, which thus becomes surrounded by the ImxIv- 

 cavity or coelom {D). The cavities are separated by the double 

 23artition-walls of mesoblast. These partitions are the dissepi- 

 ments, and the cavities themselves constitute the c(elom. The 

 outer mesoblastic Avail of each cavitv is known as tha so?ii at ic 

 layer (.§.??!.); it unites with the ectoblast to constitute the body- 



