PROCESS OF DEVELOPMENT IN BATRACHIAX OVA. 513 



the whole thickness of the back. Sufficiently thin sections of 

 preparations hardened in chromic acid permit two layers of 

 unequal thickness to be very distinctly seen in this part of the 

 dorsum. The thinner external layer is composed of small cells, 

 and is the analogue of the sensorial lamina of Remak ; the 

 thicker internal layer is composed of large cells, but which are 

 constantly becoming smaller than the large germ cells occupy- 

 ing the centre of the floor of the cleavage cavity. From this 

 internal and thicker layer an innermost unicellular layer 

 separates (intestinal gland layer, Darmdriisenblatt, Remak), 

 whilst the remainder forms the moderately thick middle or 

 motor germinal lamina. 



Fig. 400. 



Fig. 400. Vertical enlarged section of an egg of Bufo cinereus. The 

 cavity or fissure proceeding from N, and extending to z, is, for the 

 sake of clearness, drawn throughout its whole extent as a broad 

 fissure, which, however, in reality only holds good for the upper 

 half ; D, roof ; z, floor of the cleavage cavity ; F, P, white area. 



The sickle-shaped (as seen on section) mass of cells (z) con- 

 stantly rises towards the pole, and the (in horizontal sections) 

 semilunar fissure extends in the same direction. But as the 

 fissure penetrates into this newly developing mass of cells the 

 portion external to the fissure forms a definite deposit upon 

 the roof of the cleavage cavity, whilst the other part, which is 

 the thinner of the two (D) remains as a septum between the 

 fissure (N) and the cavity of Baer (F). 



If meridianal sections dividing the back into two halves be 

 made through ova at this stage of development (fig. 401), it 

 appears that those cells which have reached from the floor of 



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