382 EMBRYOLOGY 



ments from each other. Thus the germ bands can be in- 

 terpreted as being formed of both kinds of elements 

 (Whitman). As we have seen, each germ band is composed 

 of four superficial rows of cells and a more deeply located 

 one (Fig. 151 C). The ventral chain of ganglia arises from 

 the innermost row of each germ band. The cells multiply, 

 and in this way a cord of cells consisting of several layers is 

 formed from the single row. A segmentation takes place in 

 this from in front backwards. In addition, median and 

 lateral parts are differentiated in the separate cell-masses, 

 and both cords unite in the middle line. In this way arise 

 the ganglia and the commissures. 



Bergh, like Whitman, also derives the ventral chain of ganglia from 

 the germ bands, but, according to him, the permanent epidermis arises 

 from the same source, and consequently the nervous system takes its 

 origin beneath this. 



Ncsbaum's (No. 75) theory of the origin of the nervous system differs 

 from that described. He derives the ventral chain of ganglia, as well as 

 the brain, from a thickening of the ectoderm — that is to say, from the 

 primitive epidermis — and thus adopts an interpretation that (more in 

 harmony with theoretical considerations) was also espoused by Kowa- 

 LEvsKY and Balfour. The statements of Nusbaum on this and other 

 developmental processes of the Hirudinea harmonize so little with the 

 statements of the other authors on this subject that any further con- 

 sideration of them must be omitted. 



The development of the supra- oesophageal ganglion is 

 initiated in the head germs, underneath the layer which 

 supplies the epidermis, by the segregation of a compact 

 mass of cells, in which the Fuiiktsuhstanz can soon be 

 recognized (Bergh). The fundaments of the brain and 

 ventral chain of ganglia would then be distinct, and not 

 until after the concrescence of the head germs and trunk 

 germs would they be united by the development of the oeso- 

 phageal connectives. 



The Nephridia. — According to Whitman, the nephridia 

 arise from the two middle cell-rows of each germ band,^ and, 

 in fact, Whitman, in opposition to Bergh (comp. supra, p. 



^ The fate of the fourth, outer row of cells remained unknown to 

 Whitman. 



