DISCOPHORA. 349 



in question really do give rise to the nervous system he has not thought fit to 

 produce it in his paper. He figures a section with the eight neuroblastic cells 

 in the middle ventral line, and in the next stage described the nervous 

 system is divided up into ganglia ! The first stage, in which the so-called 

 nervous system has the form of a single row of eight cells, is quite unlike 

 any rudiment of the nervous system such as is usually met with in the 

 Chastopoda, and not a single stage between this and a ganglionated cord is 

 described or figured. Whitman, whose views seem to have been influenced 

 by a peculiar, and in my opinion erroneous, theory of Rauber's about the 

 relation of the neural groove of Vertebrata to the blastopore, does not seem 

 to be aware that his determination of the fate of his neuroblasts requires any 

 special support. 



He quotes the formation of these parts in Euaxes (vide preceding 

 Chapter, p. 324) as similar to that in Clepsine. In this comparison it 

 appears to me probable that he may be quite correct, but the result of the 

 comparison would be to shew that the neuroblasts and mesoblasts composed 

 together a mesoblastic band similar to that of the Oligochaeta. Till more 

 evidence is brought forward by Whitman or some other observer in support 

 of the view that the so-called neuroblasts have any share in forming the 

 nervous system, they must in my opinion be regarded as probably forming, 

 in conjunction with the mesoblasts, two simple mesoblastic bands. Kowa- 

 levsky has moreover briefly stated that he has satisfied himself that the 

 nervous system in Clepsine originates from the epiblast a statement which 

 certainly could not be brought into harmony with Whitman's account. 



ISTephelis Nephelis will form my type of the Gnathobdellidae. The 

 segmentation of this form has not yet been thoroughly investigated, but 

 Biitschli's (No. 359) observations are probably the most trustworthy. 



The ovum first divides into two, and then into four segments of which 

 two are slightly smaller than the others. Four small cells which form the 

 commencement of the epiblast are now formed. Three of them are derived 

 by budding from the two larger and one of the smaller of the four cells, 

 and the fourth from a subsequent division of one of the larger cells 1 . 

 The three cells which assisted in the formation of the epiblast cells again 

 give rise each to a small cell ; and the small cells so formed constitute a 

 layer underneath the epiblast which is the commencement of the hypoblast, 

 while the cells from which they originated form the vitelline spheres. 

 Shortly after the formation of the hypoblast, the large sphere which has 

 hitherto been quiescent divides into two, one of which then gives rise 

 in succession to two small epiblastic elements. 



The two large spheres, resulting from the division of the originally 

 quiescent sphere, next divide again on the opposite side of the embryo, 

 and form a layer of epiblast there ; so that there is now on one side of 

 the embryo (the ventral according to Robin) a layer of epiblast formed 



1 Doubts have been cast by Whitman on the above account of the origin of the 

 four epiblast cells. 



