2^4 



C, O. WHITMAN, 



detailed and accurate study it has been found to end in a blas- 

 tula and not in a morula. "A solid sphere of indijf'ereni cells'^ 

 is, to say the least, a very improbable form, so improbable that 

 its existence may be held questionable until established by 

 positive evidence. The doubt is all the more justifiable, as 

 more careful investigation has, in many cases, already shown, 

 that the so-called mulherrij- stage is not a morula, but a blastula 

 or even a gastrula. 



What is the origin and signification of the blastocoel ? 

 Baer C-^^-y) supj)osed this cavity to be the ])lace originally 

 filled by the germinal vesicle. According to Hiickel (-j'J?_v; 

 ^\), it is a cavity formed by the collection of a fluid in 

 the centre of the so-called morula. What this fluid is, where 

 it comes from, and why it appropriates a central position, 

 forcing the cells into the periphery, we are not informed. I 

 will give some reasons for the opinion that the blastocoel, lohen- 

 ever it ^jyp ears, forms as a necessari/ result of the cleavage process. 

 As is well known, a dividing cell lengthens in a direction at 

 right angles to the plane of cleavage. An interesting phenomenon 

 follows the close of the cleavage, viz. the approach of the 

 two cleavage products, in consequence of which a sphere, com- 

 posed of two distinct hemispheres, is formed (fig. 16). This 

 phenomenon is familiar to all who have followed the cleavage- 

 process. Some authors have contented themselves with a simple 

 report of the fact, while others have attempted to find an 

 explanation in the confinement of the egg in a membrane. 

 This explanation, however, cannot apply in cases where the 

 membrane stands at so great a distance from the ^^^ that it 

 ofl'ers no resistance to the complete separation of the division 

 products. 



I once, accidentally, while endeavouring to separate the eggs 

 with a dissecting needle, freed an egg from its membrane. The 

 egg divided, and the two spherical parts, at the close of the 

 division, touched each other at a single point. Immediately 

 after this they began to approach, flattening against each other, 

 and finally they formed a single sphere, as perfect as that in fig. 

 16.^ This convinced me that the membrane had nothing what- 

 ever to do with the phenomenon. I am unable to give anything 

 more than a hypothetical explanation. The cause of the 

 separation and of the subsequent approach is undoubtedly 

 the nucleus. If we suppose that the two poles of the am- 

 ])hiaster are similar poles, they will of course repel each other. 

 To account for the ajjproach, it is necessary to assume that 

 subsequent to the division one nucleus becomes positive and the 

 other negative. Tiie })roof that this is an electrical ])henomenon 



' Tlie first cleavage of the egg of Lymiiaus, or of Plauorbis, is per- 

 formed in the same muuucr. 



