274 



SCIENCE. 



[Vol. I., No. 10. 



germinal matter, protoplasm of the egg, is the 

 self-motile part. The j-elk or deutoplasm, ou 

 the other hand, is often composed of spherules, 

 granules, plates, or oval bodies, and is convert- 

 ed by metabolic processes into the first during 

 the later stages of development. The first is 

 the fiotential part of the egg : the latter is the 

 passive and nutritive. Wherever the j-elk is 

 greatlj' in excess of the germinal matter, the 

 embrj'o is often far developed, as regards mor- 

 phological details, before the deutoplasm is 

 nearlj' all absorbed, its final absorption being 

 accomplished largely through the intermedia- 

 tion of the vascular sj'stem of the embryo ; as 

 in the ova of fishes, birds, and reptiles. The 

 greater the mass of the j'elk in proportion to 

 the bulk of the germ, the more extensive is the 

 l^ermanent displacement of the nucleus from its 

 original central position as observed in the 

 young ovicell. The displacement of the nu- 

 cleus, or germinative vesicle, would then ap- 

 pear to be due to the development of the 3"elk 

 as a deposit of material of a lower grade of 

 differentiation than the germinal protoplasm in 

 the central part of the egg, as in meroblastic 

 and centrolecithal ova, from the central portion 

 of which the nucleus has been repelled, and 

 taken up into the germinal matter. 



In the eggs of osseous fishes it is certain 

 that the protoplasm, or germinal matter, is ar- 

 ranged on the outside of the yelk, or deuto- 

 plasm, in some cases, or sends down processes 

 or a meshwork into the latter, prior to the 

 time of the formation of the germinal disk ; 

 so that the teleostean ovum actuall}' passes 

 through a' centrolecithal stage. In birds and 

 reptiles, this probably occurs during late in- 

 tra-ovarian development, as impregnation must 

 occur before encapsulation in the shell, which 

 is formed in the oviduct after the albumen, or 

 ' white,' has been added. Every grade of pro- 

 portion, from a very small quantitj' of deuto- 

 plasm up to an excessive amount as compared 

 with the germinal protoplasm, ma^' occur ; so 

 that no sharp line of demarcation exists be- 

 tween trul}' holoblastic and trulj' meroblastic 

 ova The degree of inequality in the seg- 

 mentation is therefore, generallj' speaking, 

 dependent upon the amount of deutoplasm, or 

 food-yelk, which is present, and the degi-ee to 

 which the germinative vesicle has been per- 

 manently' displaced from its central position. 

 This is, however, qualified bj' certain second- 

 ary modifications, to be discussed at the end of 

 this paper. 



This principle accounts for all the forms of 

 unequal segmentation, even including the cen- 

 trolecithal, where the peripheral segmentation 



of the germinal matter ultimatelj' displaj's the 

 working of the same principle of the repulsion 

 of the nuclei from the deutoplasm, and their 

 attraction for the outer protoplasmic segment- 

 ing stratum. It, however, explains most 

 beautifully what it is that determines the de- 

 gree of inequality between the first segmenta- 

 tion-spheres of all trulj' meroblastic ova. It 

 is therefore of fundamental importance in a 

 scheme of the primarj' laws of segmentation. 



The expulsion of the germinative vesicle 

 from the centre of an evenly segmenting egg, 

 to develop the polar cells, is not to be con- 

 founded witli the movement of the nucleus 

 towards the periphery of the ovum while still 

 in its follicle, in the large-yelked meroblastic 

 tj'pe. The distinction between these two 

 cases, I believe to be fundamental. In the 

 ovum of Ostrea, Unio, Mya, etc., the nucleus 

 at the time of the emission of the egg is still 

 approximately central in position, although 

 the ova are slightlj' meroblastic ; while in Lepi- 

 dosteus, for example, the nucleus of the nearlj^ 

 mature ovarian ovum is actually' peripheral, but 

 has not yet been broken up, or lost its form. 

 Moreover, in the holoblastic tj'pe, the nucleus, 

 after its metamorphosis and conversion, in part, 

 into the first segmentation-nucleus, is again 

 repelled towards the centre of the egg, — a phe- 

 nomenon which does not occur in any meroblas- 

 tic ovum with a germ-disk of relatively small 

 dimensions, l^'ing upon a disproportionally large 

 j-elk. This is a vital distinction, and one 

 which, as far as I am aware, has not been in- 

 sisted upon in the discussion of nuclear move- 

 ments. A few illustrative diagrams from the 

 actual subjects will make my meaning much 

 clearer. 



Fig. 1 represents an ovicell from the ovary 

 of the common eel, enlarged ninety-six times 

 to show the nucleus (w) in a nearlj' central po- 

 sition, with a ver}' large number of very small, 

 globular nucleoli adherent to its walls. The 

 surrounding plasma (p-f-d) may be taken to 

 represent both protoplasm and deutoplasm, but 

 in a still undifferentiated state. Fig. 3 is an 

 ovarian ovum of the bonj- gar (Lepidosteus os- 

 seus) , very nearl^y mature, without its granulosa 

 or follicle represented, enlarged seven times. 

 The nucleus (;*) in this section has approached 

 the surface of the egg, and is almost or quite 

 in contact with an almost homogeneous outer 

 protoplasmic layer (6) just within the zona 

 radiata (a) . Upon examining the material 

 contained within the inner edge of the pro- 

 to])lasmic layer (h) , we find that still another 

 differentiation of the egg-substance has oc- 

 curred hy which a portion (p) on either side 



