158 



INVERTEBKATA 



CHAP. 



along the back with a fine needle. In this way the ball of albu- 

 minous material filling the gut can be removed, and with care the 

 endoderm itself can be removed. What is left consists of the meso- 

 dermic bands and the underlying ectoderm with its products. This 

 can either be cut into sections or flattened out and mounted whole. 

 Since development progresses from front to back, and since in one 

 and the same specimen well-developed coelomic cavities and nephridia 

 may be found in front whilst only mother cells of the mesoderm and 

 undifferentiated mesodermic bands are found behind, the whole 

 development of many organs can be elucidated by examining a few 

 embryos of suitable age. Staff found that, in the case of this species, 

 the mother cells of the nephridia appear in the ectoderm at the 



mas ctrc 



neph 



neur 



FIG. 117. Transverse section through the ventral part of an embryo of Criodrilus lacuum. 



(After Staff.) 



coe, coelomic cavity ; l.m, first formed fibrils of the longitudinal muscles ; M.S, somite ; 

 mus.circ, ectodermic cell group destined to give rise to the outer circular muscles ; neph, ectodermic 

 cell group destined to give rise to the nephridia ; neur, eetodermie cell group destined to give rise to 

 the ventral nerve cord ; v.cil, ventral ciliated patch of ectoderm. 



hinder region of the embryo, and here act as teloblasts, giving rise 

 to strings of cells by continuous budding off of smaller cells in front 

 of them, like the mesodermic teloblasts situated internally to them. 

 There are on each side four rows of such ectodermal teloblasts, and 

 the rows of cells to which they give rise become wedged in between 

 the ectoderm and the coelomic mesoderm. The two most ventral 

 rows give -rise to the ventral nerve cord, the row lying immediately 

 outside these on each side to the nephridia, and the two uppermost 

 rows on each side to the external circular muscles. The last- 

 named teloblasts are consequently termed myoblasts. 



The longitudinal muscles and the visceral muscles of Criodrilus 

 and Oligochaeta generally, like those of Polygordius, are derived from 

 the cells of the wall of the coelom. At a later period the strings of 

 cells destined to give rise to the nephridia are broken into groups, 

 and one group is pushed into each septum which divides one 

 coelomic sac from another. Here each group grows and gives rise 



