490 REPORT OF COMMISSIONER OF FlSH AND FISHERIES. [36] 



existence of a stratum of uDsegmeiited germinal matter lying beneath the 

 disk as a thin layer continuous with that which covers the yelk. This 

 has been noticed by OSllacher, Van Beueden, Rauber, Hoifmau, and 

 Van Bambeke, and I have demonstrated the fact to my own satisfaction 

 on the ova of Clupeoids, Salmonoids, and Cyprinodonts. Van Beneden 

 is the only observer who has apparently noted it in what appears to have 

 been the egg of a Gadoid fish. While I have not demonstrated the struc- 

 ture in question in the ovum of the cod, the existence of a primitively 

 structureless yelk membrane is evidence enough of the fact, taken to- 

 gether with what 1 have demonstrated in the eggs of other species. 



The later appearance of free nuclei in the yelk membrane or yelk hy- 

 poblast would appear to warrant the inference, which has been shown 

 to be the fact by Hoffmann, that in some w-ay nuclear matter had been 

 left behind in its plasma which had been derived from the first segmenta- 

 tion nucleus, Avhich would account for the germination of blood-cells from 

 its outer surface, as witnessed by Gensch and myself. I am not, at any 

 rate, inclined to believe in the theory of the spontaneous development 

 of nuclei in this layer. 



The later phenomena of segmentation of the germinal disk cannot 

 be so well observed in the live egg as the earlier ones, in that the cells 

 become successively smaller and less distinct, until finally the whole 

 disk assumes a lenticular form and is composed of a great multitude of 

 very small cells. Each of these cells, however, when the disk is hardened 

 and stained, reveals the nuclei distinctly, .and sometimes one may meet 

 ■with a cell in the act of division with the nucleus in a condition of met- 

 amorphosis. The cells are arranged in very irregular strata, as shown 

 in Fig. 13. This stratification becomes less distinct in Fig. 14, in which 

 the epidermal or epithelial layer is developed as a somewhat thinner 

 stratum than any of the cells below. There is at this period no distinct 

 differentiation of any of the germinal or blastodermic layers, if we except 

 the epithelial differentiation of the outermost layer of the germinal disk. 



Beyond this point the ditferentiation of the germinal disk into the blas- 

 toderm, in a portion of which the embryo fish makes its appearance, is 

 very gradual. In fact every step of development is but a prelude to 

 that which is to follow, but of the hidden force or impulse which deter- 

 mines the invariable mode in which it takes place we know very little be- 

 yond the fact that it has been named heredity. With Whitman we may 

 quote Bergmann and Leuckart : " Jeder einzelne Entwickelungsmoment 

 ist die nothwendige Folge des vorausgegangenen und die Bedingung 

 des folgenden. " 



8.— Transformation of the germinal disk into the blasto- 

 derm. 



>The next event in the history of the disk is its metamorphosis into 

 the blastoderm, at one side of which the first indications of the embryo 

 make their appearance. This is not fairly accomplished until three 



