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



existence of a stratuin of urisojj^iiiciited germinal matter lying" beneath the 

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

 has been noticed by CEllacher, Van Beneden, Rauber, Hoflt'man, and 

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

 on the ova of Chii)e<)ids, Sabnonoids, and Cyprinodonts. Van Beneden 

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

 been the egg of a (ladoid lish. While I have not demonstrated the strnc- 

 tnre m question in the ovum of the cod, the existence of a jnimitively 

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

 gether "vvith w'hat 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 fa(;t by Hoffmann, that in some way nnclear matter had been 

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

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

 its outer surface, as Avituessed 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 

 \ery 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 epiderm.al 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 differentiation 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 develojiment is but a prelude to 

 that which is to follow, but of the hidden force or imi)ulse which deter- 

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

 yond the fact that it has been named hercdiiy. With Wliitman we may 

 quote Bergmanu and Leuckart: " Jeder einzelne Entwickeluugsmoment 

 ist die nothwendige Folge des vorausgegangenen und die Jiedingung 

 des folgenden. " 



8. — Transformation of the germinal dirk tn'i'o 'jhe blasto- 

 derm. 



The next event in the history of the disk is its metamoridiosis into 

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

 make their appearance. This is not fairly accomplished until three 



