OF ARTS AND SCIENCES. 43 



toplasm has taken a peripheral position ; and in so doing has avoided 

 the necessity of splitting up a large mass of passive food material. 

 The central portion of the blastodisc represents the active portion of the 

 pigmented hemisphere of the frog's ovum ; ivhile the margined portion 

 of tJie disc, together with the jyeriblast, represents the active portion of 

 the unpigmented hemisphere. This correspondence is made evident 

 by the course of events described in the sequel. The prevailing 

 opinion, that the blastodisc alone undergoes cleavage, and that the 

 periblastic cells have a different mode of origin, is entirely erroneous. 

 The only difference between the holoblastic and the meroblastic types 

 of cleavage, beyond what has just been mentioned, results from a 

 difference in the rapidity with which the cleavage advances from the 

 ectodermic towards the entodermic pole. In the frog's ovum the first 

 ck'avage-planes eventually reach the entodermic pole ; in the teleostean 

 ovum, the embryo begins to form before the cleavage reaches the 

 equator of the ovum. But if the correspondence above stated be 

 borne in mind, it will be seen that the equator of the frog's ovum 

 does not cori-espond with the equator of the teleostean ovum, but 

 with the marginal zone of the blastodisc. The ectodermic and ento- 

 dermic hemispheres are not very unequal in the ovum of the frog; 

 but in the teleostean ovum there is an enormous disparity between 

 their morphological equivalents. The polar concentration of the 

 active constituents of the ovum in the latter case has reduced the 

 ectodermic hemisphere to a polar disc, and enlarged the entodermic 

 hemisphere until it embraces nearly the whole sphere. As most of 

 the material of the entodermic hemisphere lies in the margin of the 

 blastodisc and the adjoining portion of the periblast, it is plain that it 

 takes a direct share in the process of cleavage. Remembering then 

 that the virtual equator of the teleostean ovum lies in the marginal zone 

 of the blastodisc, we shall find it possible to arrive at clear and com- 

 prehensive views respecting the entire course of cleavage, the pecu- 

 liarities in the origin and growth of the germ-layers, and the manner 

 in which the embryo is formed. 



In the mature ovum, the whole protoplasmic envelope of the vitellus 

 is charged with minute shining granules, vphich render it partially 

 opaque. A few minutes after fecundation nearly all the granules have 

 dissolved, leaving the entire ovum completely transparent. The same 

 remarkable change was observed in the ovum of the Blackfish. 



At the time of clearing up, a shadowy ring, enclosing a clear cen- 

 tral area, becomes visible at the ectodermic pole. Tilting the micro- 

 scope to a horizontal position, a profile view is obtained which shows 



