SUBSEQUENT TO CLEAVING. 79 



cells of the yolk are developed as primary and independent structures, just 

 as the central cells are ; and the only probable way in which the latter may 

 contribute to the production of subsequent sets of the peripheral cells, is by 

 providing the nutritive material, or intercellular substance, out of which 

 fresh quantities of new cells may be formed to supply the place of those 

 consumed in the successive development of the several parts of the embryo. 

 In the ovum of the young salmon, also, there is complete absence of any- 

 thing like the production of young cells within larger ones. Indeed the 

 yolk of the salmon's ovum appears to consist almost entirely of a clear 

 gelatinous homogeneous fluid, without cells or corpuscules of any kind ; the 

 only trace of cells being found at the periphery in the immediate neigh- 

 bourhood of the developing embryo.* 



The difference in the opinions, therefore, of these observers, concerning 

 the nature and ultimate condition of the two sets of cells found in the yolk 

 soon after the completion of the cleaving process, appears to consist essen- 

 tially in this : that, whereas, Reichert regards the smaller peripheral cells 

 as the offspring of the large central ones, and is of opinion that all the 

 larger cells are eventually resolved into broods of similar small ones, from 

 which the embryo and its several parts are formed, Vogt believes that the 

 peripheral and central cells are essentially different from each other, and 

 are each destined to perform a separate part in the development of the 

 embryo. The earliest formed cells of the periphery of the yolk, in the 

 opinion of the latter observer, are employed to lay down the foundations 

 of the embryo and of its principal organs, and in the immediate neighbour- 

 hood of each of these cells (but not within them), successive broods of new 

 cells are subsequently developed to supply the wants of the growing parts : 

 while the cells of the centre of the yolk, on the other hand, appear to have 

 no other purpose than to elaborate fresh nutritive material or cytoblastema, 

 out of which the above mentioned successive growth of new cells may be 

 effected. As each of these large cells discharges its nutritive contents, it 

 dissolves and disappears : being directly concerned neither in the formation 

 of any part of the embryo, nor in the production of fresh broods of cells? 

 and differing, therefore, in both these respects, from the cells of the peri- 

 pheral part of the yolk. Another important point of difference in the 

 opinions of Vogt and Reichert, as may be deduced from what has just 

 been said, is that the latter considers the whole of the yolk to be directly 

 concerned in the formation of the embryo and its several parts: while 

 Vogt refers this formation entirely to the peripheral or cortical portion. 

 In this view, Vogt is supported by several facts, especially by what he 

 has observed in the development of the embryonic salmon ; for, in the ova 

 of these animals, the yolk, as before remarked, consists almost entirely of a 

 homogeneous fluid substance in which there is no appearance of cells, ex- 

 cept at the very surface, where alone the embryo is formed : the remainder 



* Hist. Nat. des Poissons, t. i. p. 11. 



