173 



in respect to the composition of the germ -bands, and especially in 

 respect to the precise origin of the cells. Strangely enough, he has left 

 the two large «mesoblasts« entirely out of consideration, and derives 

 the sexual cells (my »segment-cells«) from the »neuroblasts«! At the 

 hind end of the embryo, are found, according to his account, four rows 

 of these sexual cells on the ventral side of each germ-band ; farther for- 

 Avard only two rows are present; and at the anterior end, only one row\ 

 This remarkable reduction of four rows to one is passed over without 

 explanation or- comment. My recent study confirms my former state- 

 ments, in regard to the origin of these cells from the «mesoblasts«. The 

 »neuroblasts« do give rise to four rows of cells in each band, but the 

 number of these cells seen in transverse section does not diminish, but 

 increases from behind forward. The origin of the sexual cells may be 

 seen to best advantage in a series of sagittal sections. 



The Nerve-chord and Nephridia. 



In my first paper, I gave an accurate account of the composition 

 of the germ -bands, and traced the origin of the cells to ten primitive 

 blastomeres, thus furnishing the needed foundation for a more detailed 

 study. I satisfied myself that the nerve -chain -was formed from the 

 products of the so-called »neuroblasts«, but I was mistaken in suppo- 

 sing that all of these products entered into its composition. 



More favorable material and better methods have shown that, of 

 the eight rows of these cells (four in each band) only the two median 

 ones give rise to the nerve-chain hi) . The lateral row [m] of each band 

 probably gives rise to muscular elements, while the two rows [neph.] 

 lying between the median and lateral rows furnish the basis of the 

 nephridial organs. The two median rows of nerve-cells are faintly 

 browned Avith osmic acid, Avhile the four rows of nephridial cells are 

 more granular and deeply browned, forming thus a sharp contrast in 

 color and general appearance. The two lateral rows are slightly stained, 

 differing but little from the nerve -cells. The development of the gan- 

 glionic chain progresses from the head backwards, so that in surface 

 views it is not difficult to trace the two simple rows of nerve-cells for- 

 ward into the fully outlined ganglia. The same has been done on sec- 

 tions, leaving no doubt as to the origin of the nerve-system of the trunk. 

 I am not able to say whether the two rows of nerve - cells extend into 

 the cephalic lobe ; but I am certain that the nerve-collar, including the 

 supra- œsophageal ganglia, is formed from cells that lie beneath the 

 epidermis, and not from a thickening of the epidermis itself. 



The epidermis overlying the nerve-cells destined to form the four 

 subœsophageal ganglia, thickens up at an early date and eventually 



