DEVELOPMENT OF THE EARTHWORM. 



199 



vermiform. The two inner rows (neuroblasts) give rise 

 to the nervous system, the next two rows on either side 

 (nephridioblasts) form parts of the nephridia, while of the 

 fourth row nothing definite is known. Each row, ending 

 behind in a single cell, widens out and deepens as it is 

 traced forwards, the neuroblasts are much further forwards 

 than the mesoblasts, with the nephridioblasts just behind 

 them. The neural and mesoblastic rows can be traced 

 round the mouth and help to form the prostomium, the 

 others fade away at the sides of the stomatodaeum. The 

 mesoblast rows grow to meet one another on the median 

 dorsal line. 



Let us sum up this complex history : 



Fertilised 

 ovum. 



Blastosph 



or 

 blastuh 



with primitive 

 mesoblasts. 



Epiblast 



or 

 Ectoderm 



'(a) The original outer layer 



becomes the epidermis. 

 The secondary inner strat- 

 um consists of two rows of 

 neuroblasts which form 

 the nervous system, and 

 of four rows of nephridio- 

 blasts which form parts 

 of the nephridia. 



Mesoblast 



" mesoblasts." 



Lining of 



General Development of the Organs. Though it will involve a slight 

 repetition, we shall now describe the origin of the various organs. 



The skin arises from the original outer wall of the gastrula. The 

 "setigerous glands," within which the setts develop, and from which 

 they push their way to the exterior, arise partly from the rows of cells 

 started by the nephridioblasts, and partly in all probability from the 

 outermost of the four cell rows previously mentioned. The double 

 ventral nerve cord arises from the neuroblasts. The two cerebral ganglia 

 originate, according to Kleinenberg, independently of the ventral cord 

 from a median unpaired apical plate of ectoderm, while according to 

 Wilson they arise along with the ventral cord, and have their founda- 

 tions in the thickened anterior end of each of the two neural rows. 



The history of the excretory system is complex, (a) At the anterior 

 end of young embryos, a group of ectoderm cells, dorsal in position, 

 forms a larval excretory organ, which wholly disappears in later stages. 

 (b] Next appear two ciliated canals in the anterior region, closed inter- 



