DEYELOPMENT OF THE CTENOPnORA. 95 



Fol's dissertation, however, is mainly occupied with an account 

 of the anatomy of adult forms ; and the memoir is throughout 

 illustrated by well-executed figures of development and struc- 

 ture. 



The series of researches which, next in point of time, has con- 

 tributed most to our knowledge of the development of the Cteno- 

 phora, we owe to Alexander Agassiz, who has given us a well- 

 illustrated and valuable memoir on the embryology of Idyia 

 (JSeroe) and of Pleurobrachia *. In his account of the structure 

 of the egg and of its development, he has confirmed in most 

 points the results obtained by Kowalewsky, Fol, and myself, 

 demonstrating by independent observation the remarkable pro- 

 cess by which the first formed cleavage-spheres become gradually 

 enveloped by a laj^er of smaller spheres resulting from a later 

 division of those previously formed f. 



He has studied in great detail this process, and describes the 

 eight segment-spheres first formed as arranging themselves round 



* Alexander Agassiz, " Embyology of the Oteuophora," Mem, of American 

 Acad, of Arts and Science, vol. x. 1874. 



t It may be a question how far the enveloping layer of small spheres represents 

 a continuation of the proper yolk-cleavage, or is to be regarded as a post- 

 morular formation. Agassiz, if I have correctly interpreted his remarks, would 

 seem to maintain the latter view ; and with this I am disposed to agree, though 

 it is difficult to say where in the Ctenophora yolk-cleavage ends and a proper 

 oi'gan-formation begins, or what stage in the development of the egg we are to 

 consider as corresponding to the proper mulberry-like condition of other ova. 

 If we regard the proper yolk-cleavage as coinciding with the division of the 

 egg-cell only so long as this continues uniform, we shall have a definite character 

 to rest on, and the formation of the small peripheral spheres will then be a 

 post-morular phenomenon resulting in the formation of the two germinal layers, 

 ectoderm and endoderm. The mulberry-like condition would, it is true, in 

 accordance with this view be completed at a much earlier stage, and be repre- 

 presented by a much smaller number of cells than perhaps in any other ascer- 

 tained instance ; but, notwithstanding this, the whole process could be brought 

 more into accordance with what we already know of yolk-cleavage and its 

 immediate results than would be possible if we were to regard the formation of 

 the small peripheral spheres as a direct continuation of tlie proper yolk- 

 cleavage. 



The formation of the peripheral layer of small dark spheres by which the 

 central large clear spheres become enveloped results in a condition wliich must 

 be carefully distinguished from that of the unsegmented ovum, in which we 

 have also a central clearer mass surroimded by a peripheral darker layer. 



