PROCESS OF CLEAVAGE IN BATRACHIAN OVA. 509 



developed. On this account I shall term them germ cells (Keim- 

 zellen), and also point out that their ultimate destiny has not as 

 yet been ascertained. 



At the point where the germ cells are exposed, they are 

 at a very early period sharply separated from the outer brown 

 mantle-zone of smaller cells by a semilunar fissure (N). This 

 fissure is named after its discoverer, Rusconi's fissure.* It 

 subsequently becomes completed into a circular groove, and 

 the now still more perfectly circular and well-defined mass of 

 large germ cells has been named by Eckerf the vitelline plug, 



Fig. 399. 



Fig. 399 represents a meridianal section of an egg of Bufo cinereus, 

 the stage of development of which does not quite correspond to that 

 described on this page. F, Cleavage cavity ; D, the roof ; P, white 

 area at the inferior pole ; Z, germ cells on the floor of the cleavage 

 cavity ; z, germ cells which project from the floor of the cavity 

 towards the mantle ; N, section of Rusconi's furrow ; jR, dorsal 

 half ; jB, abdominal half of the ovum. 



(Dotter-propf). I have named that half of the eggj where the 

 semicircular furrow commences, the dorsal half, because it 

 forms the dorsum of the embryo ; the longitudinal axis of the 

 dorsum runs from the centre of this furrow to the superior 

 pole. The opposite half I have termed the abdominal half. 



It was long ago known to v. Baer that the ova underwent in 

 water a rotation of about 90, so that the meridian became an 



* Developpement, etc. 



f Icones' Physiologicce. 



1 Zeitschrift fur wisseiiscliaft. Zoologie, Band xi. 



