DEVELOPMENT OF THE ORGANS. 85 



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

 (or Uastosphere) (A, B, Fig. 35). 



The formation of the GERM-LAYERS is one of the most im- 

 portant and significant processes in the whole course of develop- 

 ment. Germ-layers like those of Lumbricus, and called by 

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

 mals ; and it will hereafter appear that this fact has a profound 

 meaning. 



Development of the Organs. (Organogeny.) The embryo gradu- 

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

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

 end, which 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 swallowed by the 

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

 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 (m, Fig. 36). 

 The mesoblast is in the form of two bands lying 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, J?, C. The mesoblastic bands are at first 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. A 

 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 (C", Fig. 35), until finally they meet above 

 and below the archenteron. The cavities at the same time 

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

 the archenteron, which thus becomes surrounded by the body- 

 cavity or co3lom (Z)). The cavities are separated by the double 

 partition-walls of mesoblast. These partitions are the dissepi- 

 ments, and the cavities themselves constitute the co3lom. The 

 outer mesoblastic wall of each cavity is known as the somatic 

 layer (s.m.); it unites with the ectoblast to constitute the body- 



