THE EGG AND ITS MEMBRANES. 45 



spicuous nucleoles. Cell boundaries are sometimes better seen than in earlier 

 stages, and large vacuoles are present near the tunic's outer wall ; surrounding this 

 the ovarian stroma is reduced to practically a single-celled layer, which now alone 

 separates the tunic from the blood in the large gonadial sinus (fig. 33, gs). Com- 

 parison with earlier stages thus indicates — paradoxical as the statement reads — that 

 the arterial supply of the developing egg is progressively diminished and the venous 

 supply progressively increased.* 



The foregoing details are given, since they indicate the complexness of the 

 problem of the growth of the egg in Chimsera. There here exist at various stages 

 not a tunic of an almost unvarying character, as apparently is the case in sharks, 

 but one which in earlier stages is shark-like, but which later changes progressively, 

 diminishing its thickness and reducing the number of its component elements, to 

 the end that each cell of this membrane comes in immediate contact on the one 

 hand with the egg and on the other with the nutritive fluid. It is further clear that 

 the elements of the tunic acquire changed physiological characters as development 

 proceeds — witness the changes which occur in size, shape, and disposition of the 

 nuclei, the appearance of vacuoles in the late stage — pari passu with changes in the 

 arrangement of the blood supply. 



YOLK. 



The yolk masses at first occur in the granular ooplasm close to the zona; next 

 they appear in vacuoles, cavities which are noted before the appearance of the yolk 

 masses, and are later seen to become greatly enlarged and to be drawn together 

 around the masses of yolk. The yolk itself increases in bulk, its masses now often 

 presenting irregular protuberances, resulting apparently from a process of accretion. 

 In surface view many of the yolk masses appear botryoidal (in eggs preserved in 

 sublimate, acetic-sublimate, picro-sulphuric, picro-formalin). And this condition 

 persists while the egg is attaining its mature size. In stages as late as gastrulation 

 the coarse yolk differs little outwardly from the foregoing conditions. The grains 

 show only a smoother surface and a possible tendency to coalesce ; but it is 

 evident that the vacuoles are now more closely adjusted to the yolk. The fine 

 yolk, on the other hand, is, as Riickert's figures indicate in sharks, derived from 

 the coarse yolk by a process of subdivision. Comparing the earlier stage (fig. 32) 

 with one at fertilization (fig. 35), we observe that the substance of the former grains 

 has become subdivided into morula-like masses of minute deutoplasmic elements, 

 these, as before, lying in large vacuoles. In each of these masses one notes that 

 there has usually been produced a globule of a highly refringent substance analo- 

 gous to the oil-drop of the teleostean egg. In a later stage the corresponding por- 

 tion of the egg has become a well-defined region of germinal yolk (Riickert's Keim- 

 dotter), and we are led to conclude that the later condition, with fine grains of yolk, 

 is the result of a continued process of subdivision of the morula-like masses and 

 their subsequent confluence. (The general character of the germinal yolk is shown 

 in figs. 33 A and 11 b at^j'.) 



* In its latest stage the ovarian egg shows a series of capillaries (plate 11, fig. 5), converging to an elliptical stigma. 

 Unfortunately, the relations of the tunic in this stage were not examined. 



