OVUM. 



73 



colour, and in the boiled egg by its softer 

 consistence and less granular appearance, is 

 found by microscopic examination to consist 

 of organised corpuscles floating in a larger 

 portion of fluid, and different from those of 

 the external part of the yolk. The transition 

 from the one kind of corpuscles to the other 

 in these two portions of the yolk, is not 

 sudden ; but many gradations of intermediate 

 forms are to be met with on the confines of 

 the two regions. 



In the central part of the cavity or latebra, 

 which, when boiled, appears like a thick milky 

 fluid, corpuscles very different from those of the 

 external part are to be found (see fig. 52, c). 

 They are almost all of a very regular spherical 

 form with a delicate and clear, but distinct 

 vesicular wall ; the interior of the vesicle is 

 occupied by a perfectly limpid fluid, and by 

 one or several highly refracting globules of 

 various sizes, not exactly similar to nuclei, but 

 rather like oil globules, floating within the 

 cell and moving with freedom from one part 

 of it to another. The diameter of the clear 

 vesicles varies from T ^ to -ji^ of an inch, the 

 most being about -g^- ; therefore about half 

 the size of the granular corpuscles of the 

 yellow yolk. The internal oil globules are of 

 very various sizes, the largest being generally 

 about a third or a fourth of the diameter of 

 the enclosing vesicle. Mingled with these 

 vesicles, there are also floating in the fluid of 

 the yolk cavity in considerable numbers, but 

 in less quantity than the vesicles them- 

 selves, a set of simple highly refracting 

 globules, exactly similar to those contained 

 within the vesicles from which we may sup- 

 pose they have been set free. These oil-like 

 globules are of every variety of size, from the 

 minutest point up to T ^Vd or ToVo f an inch. 

 Towards the surface of the yolk cavity and 

 canal, and extending below the cicatricula, 

 where the vitelline substance gradually passes 

 into the darker yellow yolk, the microscope 

 shows some mixture of and transitions be- 

 tween the several cells or corpuscles before 

 described, those of the intermediate structure 

 being in greatest numbers ; these exhibit very 

 various gradations of deposit within them, 

 from the finest granular particles in some, to 

 larger and fewer oil-like globules in others. 

 In most of these transition corpuscles a delicate 

 vesicular wall is perceptible. In the more ad- 

 vanced of these transition forms, as the minute 

 granules are in the process of uniting into 

 larger and larger oil globules, and at last coa- 

 lesce into a very few or into a single one, the 

 condensation of the exterior layer increases to 

 form a vesicular wall, and a separation of an 

 albuminous fluid from the oil globules takes 

 place within (see^g. 52, B). It is these vesi- 

 cular globules of the cavity which, according 

 to Reichert, are the more immediate source 

 of additions to the germinal membrane in the 

 course of development ; for the cavity and 

 canal expand, as it were, at the expense of 

 the yellow yolk, and as these inner globules 

 increase the extension of the haloes ami 

 change of colour of the yolk in the first days 



of incubation spreads rapidly over its surface 

 below the germinal vesicle. 



3. Cicatricula or proligerous disc. There 

 does not appear to be any marked difference 

 as to the minute structure of the mass of the 

 yolk and its cavity in the newly laid egg and 

 in the mature ovarian ovulum ; but the cica- 

 tricula undergoes a great change during the 

 passage of the ovum through the oviduct, 

 which is indicated in a marked manner by the 

 difference in its microscopic structure. During 

 this period, besides the loss of the germinal 

 vesicle, the cicatricula has undergone the 

 peculiar process of segmentation and cell 

 formation, upon the details of which it is 

 my intention to return in connection with the 

 special history of that process in the ovum of 

 mammalia, batrachia, and other animals. The 

 cicatricula of the laid egg is, in fact, after 

 having undergone this process, the organised 

 blastoderm or germinal membrane in which, 

 under the influence of the heat of incubation, 

 the rudiments of the embryo take their origin. 

 It already consists, before incubation, of two 

 layers of organised cells, which are the indica- 

 tion or earliest condition of the upper or serous, 

 and lower or mucous layers, which were de- 

 scribed by Pander and Von Baer as taking 

 their origin only after incubation for some 

 hours.* (Seey?g. 52, D.) 



The cells of the upper layer are about - T rr3 

 of an inch in diameter. They are closely set 

 and very slightly connected together in a 

 continuous layer one cell thick, presenting a 

 smoother upper surface next the vitelline 

 membrane. Each cell consists of an external 

 vesicular wall, a distinct nucleus, and some 

 granular deposit. The nucleus is highly re- 

 fracting. The cells of the lower layer are 

 nearly double the size of the upper ones, more 

 regularly spherical and less closely connected 

 together. They do not in general present any 

 single nucleus, but rather a small mass of 

 granules and oil-like spherules within them, 

 giving them much of the appearance, though 

 smaller in size, of the corpuscles found be- 

 tween the cavity and rest of the yolk. 



In the cicatricula or proligerous disc of the 

 ovarian yolk, on the other hand, containing 

 the germinal vesicle set in its centre, the 

 microscope shows no truly organised cells, 

 but only a mass of simple spherules of very 

 various sizes, but the largest of which for 

 the most part arc less than half the diameter 

 of the cells in the upper layer of the blas- 

 toderm of the laid egg. They are without 

 any nucleus, and have all the appearance of 

 simple solid spherules from ^L- to ^oW of 

 an inch in diameter, of considerable ref'ractin"- 

 power, and, indeed, very similar to the nuclei 

 of the cells in the upper blastodermic layer. 



Vitelline membrane. The condensed layer 

 of structureless membrane which has gene- 



* The most exact descriptions of the minute 

 structure of the cicatricula are those of Schwann in 

 his Microscopic Researches; of Reichert, in his 

 Beitrage zur heutige Entwickelungsgeschichte, c. ; 

 and Remak, in his Beitrage zur Eutwick. des 

 Iliilmchens, &c., 1850. 



