g 6 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



the germinal wall, next having just emerged from the germinal wall, and finally 

 having almost passed through the niveau of the yolk-entoderm. We have even 

 data indicating how the upward migration of such a megasphere takes place. One 

 sees in L a line of vacuoles appearing between the megasphere and the yolk- 

 entoderm, and it follows clearly that the vacuoles, by a process of coalescence, 

 provide a less resisting space into which the megasphere can rise. 



Regarding, in the next place, the fate of the megaspheres, I think that there 

 can be no doubt that they serve to bear nutriment to the tissue which they enter. 

 In some cases, as in fig. 71 p, at y, they undergo mitotic division (after having divided 

 only by amitosis in the yolk), and their descendants can not be distinguished from 

 the neighboring cells. In other cases, M and N, they become closely surrounded 

 by cells, entoderm in the present case, which form around them a syncytium, and 

 appear to serve as nutriment distributors; witness for example the grouping of the 

 cells around the large megasphere in M, and the radiating arrangement of the cells 

 adjacent to the cluster; even the ectoderm is budding off a cell at the point nearest 

 the megasphere. 



In a word, I think we can fairly conclude that in Chimaera, even in this late 

 stage, cells are constantly being added to the blastoderm from the germinal wall. 

 This condition maintains in the case of the megaspheres, as we have just noted, 

 and it holds equally good for other types of cellular additions to the blastoderm. 

 We thus observe in o (a detail of section G) that between the yolk-entoderm (yc), 

 and the wall of fine yolk (y) is a vacuolar zone,* in which merocyte elements are 

 being ferried over to become cells of the blastoderm; thus at v' is a vacuole into 

 which the merocyte (m) is about to pass. It is to be noted, however, that cells 

 may also appear in the finer yolk, and thence by the mediation of an enveloping 

 vacuole be passed upward into the vacuolar zone, thence to the blastoderm (cf. in 

 fig. 710, at /). 



That throughout these stages there is a general transformation of the yolk 

 from coarser elements into finer elements there can be no question. Deep in the 

 yolk appear nuclei surrounded by spherical masses of finer yolk, in turn surrounded 

 by masses of coarser yolk, in turn more or less irregularly by a system of vacuoles 

 (= intercellular spaces) fig. 71 K. There is, to be sure, a greater or less amount 

 of coalescence of these yolk elements, and in the zone close to the entoderm we 

 observe that the nuclei with their surrounding fine yolk have come to merge into a 

 single layer (= the zone of merocytes of the subgerminal wall). It is from the 

 elements of this layer in turn that some cellular additions to the blastoderm are 

 made. 



The nuclear changes which occur during the process of their "levitation" are 

 worthy of especial comment, for while the cells of the yolk-entoderm now divide by 

 mitosis (as in o), the nuclei of the region below the vacuolar zone divide amitotically, 



*Similar conditions have been observed in the early stages of teleosts (cf., among others, Hoffmann, Zeit. wiss. 

 Zool., vol. XLVI (1888), pi. xxxv, a paper, by the way, which is too little referred to in recent work on teleostean 

 embryology). 



