150 THE BIOLOGY OF AN ANIMAL. 



In some species of Lumbricus the mesoblast is in part formed by a mul- 

 tiplication of the ectoblastic cells at their inner ends. In other species no 

 large primary mesoblastic cells can be distinguished. 



The appearance of the ectoblast, entoblast, and mesoblast is 

 known as the formation (or differentiation) of the GERM-LAYEKS, 

 and is one of the most important and significant processes in 

 the whole course of development. Germ-layers like those of 

 Lumljricus, and called by the same names, are found in the em- 

 bryos of all higher animals ; and it will hereafter appear that this 

 fact has a profound meaning. 



Development of the Organs. The embryo gradually increases in 

 size and at the same time elongates. As it lengthens, the blasto- 

 pore (in this case the moutK) remains at one end, which is there- 

 fore to be regarded as anterior, and the elongation is backwards. 

 The two primary mesoblastic cells are carried backwards, and 

 always remain at the extreme posterior end (i,j, Fig. 85). 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 mesoblast is in the form of tw r o 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. 85, A, A 1 . The mesoblastic bands are at first solid (A), but 

 after a time a series of paired cavities appears in them (j), 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 

 in A', /, Fig. 85. As the bauds lengthen they also extend up- 

 wards and downwards (/, Fig. 85). until finally they meet above 

 and below the archenteron (m~). 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 coelom. The cavities are separated by the double partition- 

 walls of mesoblast. These partitions are the dissepiments, and 

 the cavities themselves constitute the coslom. The outer meso- 

 blastic wall of each cavity is known as the somatic layer (s.w.); 

 it unites with the ectoblast to constitute the body- wall (somato- 



