IN INVERTEBRATE ANIMALS. 69 



II. In the second variety of the process of yolk-cleaving, only a part of 

 the yolk is the seat of this phenomenon. The only invertebrate animals in 

 which this variety has been observed, are the Cephalopods, and a description 

 of the process as it occurs in the ova of these animals has been furnished 

 by Kb'lliker from observations on the Sepia.* In the unimpregnated ovum 

 of this animal, as in those of fishes, and Alytes obstetricans,f the germinal 

 vesicle is situated at one part of the surface of the yolk, instead of being 

 imbedded in the centre, as it is in most other cases. And the pro- 

 cess of cleaving commences in the situation of this vesicle, and in 

 the Sepia, is confined to its immediate neighbourhood. Shortly after 

 the disappearance of the germinal vesicle, consequent on fecundation, 

 a slight elevation of the yolk appears at that part of the surface where 

 the vesicle was situated. This elevation soon divides into two promi- 

 nences, within each of which is contained a nucleated cell, surrounded 

 by granular matter. The. two prominences are shortly divided into 

 four, and then into eight, segments, each containing a cell surrounded 

 by granules. These segments have the form of segments of a circular 

 disc, all meeting with round, well- denned, prominent ends at the centre, 

 where the nucleated or embryonic cells are situated ; but at the periphery 

 of the circle, passing, without any definite line of separation, into the 

 yolk. At the next stage the eight segments of the circle give off at 

 their apices eight globular segments, which form a ring within the 

 radiating segments. Each globular segment, as well as each newly- 

 formed apex of the radiating segments, contains an embryonic cell. All 

 these segments then divide, so that there result sixteen radiating, and 

 sixteen globular segments, in each of which is an embryonic cell. A new 



next stage of the process, the four spherical masses, instead of dividing into eight smaller 

 equal-sized spheres, retain, as nearly as possible, their original size, while from one surface 

 of the crucial- shaped plane which they form, four other spherical bodies gradually arise, 

 which, when fully formed, are about half the size of the large ones, and much more trans- 

 parent, owing to the very few granules which they contain ; each possesses a clear vesicle 

 similar to that in the larger spheres. It did not appear as if these smaller masses were 

 formed by a division of the larger spheres, for, as above stated, these latter maintained 

 almost their original size ; they seemed rather to be produced by a kind of exudation from 

 the large masses of the more viscous part of the yolk- substance, few granules entering into 

 their composition. After this event, the several segments of the yolk go on dividing and 

 subdividing in the ordinary way, and the difference in size between the two above-mentioned 

 sets of spheres shortly disappears ; but throughout the whole process the divisions of the 

 larger set remain granular and opake, while those of the small set continue to be charac- 

 terized by their transparency. When the process of cleaving is complete, the former are all 

 found in the centre of the yolk, and are accordingly named by Vogt the central spheres, 

 while the latter are situated at the surface, and are hence termed by him peripheral spheres. 

 From the central opake spheres, or cells as they have by this time become, he states that the 

 central parts of the embryo are formed while the peripheral organs are developed out of the 

 transparent spheres or cells. 



* Entwickelungs. der Cephalopoden, pp. 1 7 40. 



t Entwickel. der Geburtshelferkrote, p. 1. 



