1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 515 



greatly diminished ; it forms only a single layer of cells, and the 

 pedicle which connects the body with it is almost entirely composed 

 of stroma-ingrowths, which form for the most part a very lax net- 

 work of fibres and cells, thongh denser in the middle, and sur- 

 rounding a slender cord of germinal cells, the sole remnant of the 

 former epithelial pedicle. 



In a somewhat more advanced stage there is an identical structure 

 with that just described as regards the specialization of the peri- 

 pheral layers and the mass of central cells ; the formation of yolk 

 has, however, gone on much faster, and the follicular cells, as well as 

 most of the cells of the central mass, are crammed with variously 

 sized yolk- spherules. 



Here and there, especially in the periphery of the central cells, are 

 irregularly shaped masses of yolk-spherules (fig. 24, a), among which 

 are occasionally discernible nuclei like those of the surrounding cells. 

 A comparison of Plate LIV. fig. 24 with Plate LIII. fig. 1 1 will show 

 how very similar in size and extent these irregular patches of yolk 

 are to the patches of amorphous deeply staining protoplasm in tlie 

 earlier stage ; and 1 cannot but think that they are these same 

 patches of protoplasm produced by the fusion of some of the central 

 cells which have commenced to form yolk-particles on their own 

 account. 



Other preparations, again, seem to indicate that the fusion of 

 certain of the central cells either does not take place at all, or takes 

 place after the formation of the yolk. I have a nearly complete 

 series of sections through an ovum in which the yolk has already 

 commenced to be formed ; the cells of the follicular epithelium are 

 full of yolk-particles, as are also the central cells. The latter form 

 a compact mass of cells containing abundant yolk-particles, but 

 without any definite patches of yolk lying between the cells, sucli as 

 could be compared to the patches of protoplasm formed by a fusion 

 of cells. In many sections, however, of this series it happened that 

 the central mass of cells had dropped out, so that I cannot make any 

 positive statements about the point of discussion raised. This par- 

 ticular instance showed very plainly indeed the connection of the 

 mass of central cells with the follicular epithelium ; the area of con- 

 nection was very limited, as has been already mentioned in other 

 cases. 



The central cells are united with the follicular epithelium by a 

 bridge of tissue which has every appearance of being an outgrowth 

 of the latter ; the nuclei are round and pressed close together as if 

 in a condition of active multiplication ; they pass without any break 

 into the follicular epithelium, Irom the cells of which they cannot, 

 indeed, be distinguished, and gradually on the other side into the 

 mass of central cells. The cells of the latter have the ordinary 

 characters that have been already described. I noticed a very large 

 number of pecuhar cells, several of which are illustrated in fig. 4 ; 

 similar cells are not absent in other cases, but I never have seen 

 them so numerous as in the present case. These cells are easily 

 to be distinguished from the germinal cells among which they lie 



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