DEVELOPMENT OF THE EARTHWORM. 



175 



daughter cells behind them. The cells so formed also divide, 

 the embryo rapidly lengthening and finally becoming ver- 

 miform. 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 stomatodasum. The 

 mesoblast rows grow to meet one another on the median 

 dorsal line. 



Let us sum up this complex history : — 



FtrtiUsed Blastosphere 



blastula. 



Two-layered 



gastrula 



with primitive 



mesoblasts. 



Epiblast 



or 

 Ectoderm 



-; 



''(«) The original outer layer 

 becomes the epidermis. 



ih) 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 



Mesoderm 



formed from J Blood-vessels. 

 the division of I i""=' '^f'\°^ nephridia. 



the primitive (^Reproductive organs, 

 'mesoblasts.' 



Hypoblast I Lining of 

 Endoderm.l™'^-S"'- 



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 jrfiZ 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 

 is likewise ectodermic, though somewhat indirectly so, aris- 

 ing as it does from the neuroblasts. The two cerebral ganglia 



