DISCOPHORA. 289 



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 Oligochseta. 

 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 niy opinion be regarded as pro- 

 bably forming, in conjunction with the mesoblasts, two simple mesoblastic 

 bands. Kowalevsky has moreover briefly stated that he has satisfied himself 

 that the nervous system in Clepsine originates from the epiblast a state- 

 ment which certainly could not be brought into harmony with Whitman's 

 account. 



Nephelis. Nephelis will form my type of the Gnathobdellidse. 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 

 of six cells, and on the opposite side a layer formed of four cells. The 

 two layers meet at the front border of the embryo and between them are 

 placed the three large vitelline spheres. The two patches of epiblast cells 

 now rapidly increase, and gradually spread over the three large vitelline 

 spheres. Except where they meet each other at the front edge they leave 

 uncovered a large part of the margin of the vitelline spheres. 



While these changes have been taking place 011 the exterior, the 

 hypoblast cells have increased in number (additional cells being probably 

 derived from the three large vitelline spheres) and fill up in a column-like 

 fashion a space which is bounded behind by the three vitelline spheres, and 

 in front by the epiblast of the anterior end of the embryo. At the sides of 

 the hypoblast the mesoblast has become established, probably as two lateral 

 bands. The origin of the cells forming it has not yet been determined. 

 The hypoblast cells in the succeeding stage arrange themselves round a 

 central archenteric cavity, and at the same time rapidly increase in size 

 and become filled with a secondary deposit of food-yolk. Shortly after- 

 wards a mouth and thick-walled oesophagus are formed, probably from an 

 epiblastic iiivagmation. The mesoblast now forms two curved lateral 



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

 epiblast cells. 



B. E. 19 



