GERM-SEGMENTATION IN MAMMALS AND BIRDS, 



079 



observers even previous to its commencement, there may be seen within 

 each mass a small clear space similar to a nucleus. To these precursors 

 of nuclei the name of hiasfidc has been given. 



In the next stage each of the two first segments becomes cleft so 

 as now to form four (c), each one of these having its clear spherule or 

 nucleus within ; a third division resolves the masses into eight, of like 

 composition with those which preceded them (d), the segments becoming 

 of less and less size in successive stages, as meanwhile the bulk of the 

 ovum as a whole undergoes little increase. The fourth stage ends in a 

 division into 16 segments, the fifth into 32, the sixth into G4 (hg. 493), the 

 seventh into 128, and the eighth into 256 (fig. 494). But it is right here 

 to observe that while it is possible in the earlier stages to trace the 

 reduplication of individual masses so that the succession of their 

 numbers, when the division is complete in each stage, follows in the 

 series of the multiples of two, yet, as the division of the different masses 

 in any stage is not simultaneous, other and as they might be termed 

 irregular munbers may be observed, especially in the earlier intervals of 

 division ; as for example, three between the first and second stage, or 

 five, six, or seven masses between the second and third, and so on. 

 In the more advanced stages, from the great increase in number, it 

 becomes almost impossible to follow the division of individual masses. 



It is also deserving of notice that while the earlier clefts seem to 

 pass right through the yolk and its first segments, so as to involve 

 in the first four or five stages the whole mass of the yolk, in the later 

 stages they do not do so, and the process seems to be in so far dif- 

 erent, that the segmenting spheres come to be collected on the surface, 

 and a mass of unsegmented granular and semi-fluid protoplasm or 

 yolk substance remains within. However this may be effected, it is 

 certain that the later division involves only the superficial set of 

 spherules, and when the process is completed, the yolk mass comes 

 thus to be covered by a layer of these protoplasmic spherules or segment 

 globules, each of which possesses a nucleus and may after a time also 

 acquire an external envelope, so as to present in all respects the features 

 of a fully formed organised cell. (See the account of the histological 

 relations of these spherules in the General Anatomy, p. 9). 



Fig, 494. 



Fig. 494. — Ovum op the Rabbit from the 

 Utekus. (from Kolliker after Bisclioff). ^'^ 



The whole surface of the yolk is now divided 

 into cellular comiiartments. A dark spot below 

 marks the position of a quantity of granular 

 spheres inside the cellular elements of the 

 blastoderm, a, the albuminous layer, now much 

 thinned out and incorporated with the zona ; b, 

 the cells of the outer layer of the blastoilerm 

 resulting from segmentation ; o, the spot of granu- 

 lar opaque spheres. 



I. Segmentation in the iird's ovum. — 

 In the ova of birds the segmenting pro- 

 cess is somewhat different from that 

 now described in mammals, seeing 



that it is restricted to the germinal disc or cicatricuui. From the 

 researches of Coste and several concurrent observations it appears that 



