!')',() REPORT OF COMMISSIONER OF FISH AND FISHERIES. [!)fij 



clcavago. In fact, it gives us a clew 1o why it is tliat there is such a 

 (listinctiou between ova as the evenly and unevenly segmenting or the 

 holoblastic and nieroblastic types. It will also be seen that it is i)rob- 

 able that the two types i)ass into each other by insensible gradations, 

 which we find is truly the fact when we come to institute a large series 

 of comparisons between observed types of development. Moreover, 

 the food-yelk has simplj' a physiological significance; it is merely a 

 store of food, which has been superadded during the intraovariau 

 growth and maturation of the egg, yet the effect of this superadded 

 yelk is to modify' the process of segmentation profoundly. That store 

 of deutoplasm which is added to the Qgg to nourish the embryo and 

 carry it to that condition of development when it can in a measure take 

 care of itself, is also of profound significance in relation to natural selec- 

 tion, by the operation of which it can alone be supposed that yelks 

 were evolved. This all-com])rehending Darwinian law is therefore seen 

 to have influenced the mode of segmentation of ova bj- and through the 

 minor and secondary law of nuclear displacement which has been indi- 

 cated above. 



The degree also to which the nucleus is transporter! from its primi- 

 tively central position determines the degree of inequality of the first 

 segmentation. It is now in the highest degree ])robable that in the for- 

 mation of the germinal disks of meroblastic ova the process is one and 

 the same throughout the animal kingdom, viz, that its development is 

 accomplished by the migration and concentration of the germinal mat- 

 ter of the egg at its animal pole. We have evidence in superabundance 

 in favor of such a view of the matter, in a great many departments of 

 embryology. There is an evident tendency on the part of the germinal 

 protoplasm of ova to separate itself spontaneously from the food yelk or 

 deutoplasm and aggregate itself either superficially over the whole 

 surface of the ovum, as in the case of centrolecithal, or at one pole mainly, 

 as in meroblastic or telolecithal types. We will find, however, that the 

 distinction between these two forms is primarily less important than might 

 be sni)[)osed, for the meroblastic type i)asses through a ilistinct centro- 

 lecithal stage prior to the development of the germinal disk in Gadus, 

 while in other forms the mode of disk development is complicated by a 

 network passing down into and between the deutoi)lasni masses from 

 the external stratum of germinal i)rotoplasm, as in the ova of Clupeoids 

 and Leuciscus. The remarkable centrolecithal segmentation and ar- 

 rangement of the protoplasm of the eggs of the arthropods is, therefore, 

 found not to be so radically dilierent from the usual type as might at 

 lirst be supposed. Its arrangement, under slightly difierent laws of 

 segmentation, is referable to the same fundamental princii)le governing 

 the dissociation and iiggregalion of the proto])lasm and (leutoi)lasminto 

 separate masses. I would also regard the deutoplasm as almost entirely 

 passive in the process of its absorption during the later stages, for we 

 Lave seen that it is actually api)roj)ri:ited by a remnant of the original 



