DEVELOPMENT. 87 



nucleus destined to take the place of the germinal vesicle. It is quite possible, however, that 

 such a nucleus may exist, though, fi'om its small size, and from being so deeply imbedded in the 

 mass of the vitcllus, it may have eluded our attempts to discover it. 



The first step observable in the segmentation-process is the cleavage of the yolk into two 

 segments (C), immediately followed by the cleavage of these into other two, so that the vitelhis 

 is now composed of four cleavage-spheres (U). In none of them, however, can a nucleus be as 

 yet demonstrated. 



The segmentation would now appear to proceed very rapidly, but not always with 

 absolute regularity ; for it would seem occasionally to advance more rapidly in some of the 

 previously formed spheres than in others. By the time that the vitellus presents about thirty- 

 six or more cleavage-spheres (E) we begin to recognise in some of these spheres a distinct 

 nucleus ; while, as the spheres become smaller and more numerous, the nuclei become more and 

 more ap]3arent, until at last there may be seen in every minute sphere, of which the segmented 

 yolk is composed (F), a brilliant nucleus, visible not only in the superficial spheres, but also in 

 the deeper ones which come into view when the ovum is broken down under the compressor. 

 It is, therefore, highly probable that in the earlier stages also a nucleus exists in every 

 cleavage-sphere, but that in consequence of the thickness and opacity of the enveloping vitellus 

 it is withdrawn from observation. The cleavage-spheres at this stage present the same peculiar 

 structure which we find in the yolk just before the commencement of segmentation, consisting, as 

 they do, of minute spherical corpuscles, with still more minute granules. When the vitellus has 

 thus become broken up into _a great number of minute spheres, it is evident that the most 

 superficial of these spheres have arranged themselves into a distinct stratum, consisting of a 

 single layer of spheres, and completely enveloping the more internal parts (G). 



We next find that the spheres composing this layer have increased in number, while at the 

 same time they have become longer in the direction of the radius of the ovum, and now form a 

 rather thick layer of undoubted cells, arranged with their long axes perpendicular to the surface 

 of the ovum, having their sides in close contact and investing, as with a continuous wall, the whole 

 interior of the mass (H). 



It is impossible not to see in the entire process here described an exact parallelism with the 

 early stages in the development of the holoblastic vertebrate ovum, while the superficial layer of 

 cells, to the formation of which we have just arrived, must — though as yet showing no obvious 

 tendency to a splitting into distinct lamintc — be at once recognised as the representative of the 

 vertebrate blastoderm. 



The nuclei, which were previously visible in the cleavage-spheres, have now ceased to be 

 distinguishable, while these spheres at the same time show a distinct investing membrane. In 

 fact, on now carefully breaking down the ovum under the microscope, its interior is found to 

 consist entirely of loosely aggregated cells, some spherical, some more elongated, and all with a 

 more or less copious endogenous brood of secondary cells within them. 



The external enveloping layer having thus attained a considerable thickness, and a well- 

 defined differentiation between it and the more internal parts having been established, the ovum 

 begins to elongate itself, and at the same time the interior has undergone a further change; for 

 we no longer find in it the large mother-cells with their- endogenous brood, but a multitude of 

 small, free, clear vesicles, of various sizes, mingled with minute granules, similar to those which 

 had all along formed a part of the constituents of the segment spheres. 



