508 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



cleavage cavity; the thin- walled dome surrounding it, and 

 formed of small cleavage elements, or embryonal cells (rind of 

 the apple), the roof (D); and the solid inferior half, the 

 floor of the cleavage cavity. 



Whilst this cavity is forming, the cleavage of the inferior 

 solid segment gradually progresses, though rather upon its sur- 

 face than in its interior. Thus it happens that the whole ovum 

 is very soon invested by a mantle of small cleavage segments 

 or embryonal cells. The resemblance to a partially excavated 

 apple is now still more striking. 



The rind of the apple corresponds to the mantle of small 

 cells, and the solid substance of the lower part to those remains 

 of the germ which only slowly undergo cleavage, and at the 

 period when the mantle is divided off still consist of very large 

 cleavage masses. The comparison, however, may be rendered 

 complete if a circular piece be cut from the rind of the spheri- 

 cal apple at its inferior pole, so as to expose the flesh ; for the 

 progressive division (fission) of the superficial cells does not 

 extend so far as to the inferior pole. At this point a small, 

 and in the first instance irregular, but subsequently circular 

 area (P) remains, the centre of which corresponds to the inferior 

 pole, and which is composed of large polygonal facet tes. 



Whilst the external surface of the mantle of all Batrachian 

 germs at this stage of development is of a dark-brown colour, 

 this area remains whitish if the lower half of the ovum was so 

 from the commencement (Bufo fuscus) ; or it becomes whitish 

 if the lower half of the fresh-laid egg was brown (Rana tem- 

 poraria, Bufo cinereus, and viridis). 



The large white cells that occupy the floor of the cleavage 

 cavity, and are exposed at the inferior pole (z, fig. 399), were 

 termed by Reichert the central vitelline mass (centrale Dotter- 

 masse). Remak had already applied the term gland-germ to 

 it (Driisen-keim), because he had found, in accordance with 

 Rusconi, that in Batrachian ova there was no structure analo- 

 gous to the vitellus. I cannot accept this nomenclature, be- 

 cause the theory on which Remak arrived at it is not tenable. 

 These cells are not exclusively devoted to the formation of the 

 rudiments of the glands. At the same time they are not lami- 

 nated cleavage elements from which the various tissues are 



