THE GENESIS OF THE EGG IN TRITON. 273 



manner described by Gegenbaur (13) seems most pro- 

 bable.i 



The persistence of bounding lines, by means of which 

 the individuality of the yolk masses is maintained for a re- 

 markably long time, makes it clear that there is a centripetal 

 growth of these masses. It is probable — we may say certain 

 — that this growth is due, not only to the expansion of the 

 individual spherules, but also to the formation of new 

 spherules, which are added to the mass on the central side.* 

 Just before the complete obliteration of the boundary lines 

 of the yolk aggregations, the entire body of deutoplasmic 

 spherules may be said to be made up of a considerable 

 number of pyramidal masses, the bases of which lie in the 

 subcortical zone, while the apices lie in the perinuclear 

 zone. The basal portions of these pyramids are formed 

 first, and the apical portions, which are farthest removed 

 from the nutritive supply, are the latest to develop. It 

 should be added, however, that the boundary lines never 

 extend to the innermost, last formed, zone of spherules ; 

 hence the outlines of the yolk pyramids are never complete. 



The distribution of the earlier formed yolk-spherules in 

 small masses or aggregations in the subcortical zone has 

 Deen observed by Gotte in Bombinato?' igneus (7, p. 17) ; 

 by Balfour (14, p. 410) in Scyllium canicula ; and by His 

 (15, p. 31, figs. 32 and 40, pi. iv) in Salmon. 



The form of the yolk-spherules is from the outset ellip- 

 soidal. In the mature egg the larger spherules measure 

 •Oil mm. by '017 mm.; they are clear and homogeneous, 

 showing nothing which could give any support to the idea 

 that they are of a cellular nature. The cortical layer of 

 protoplasm, which has a thickness of '007 mm. in fig. 23, 

 becomes very thin in the mature egg, but does not wholly 

 disappear. In fig. 26, which represents a ripe follicular egg, 

 the yolk-spherules have encroached much upon the cortical 

 layer, and here, as in earlier stages (figs. 24 and 25), the 

 size of the spherules diminishes in the peripheral portion. 



Vitelline Membrane. 



As before remarked, the primordial egg-cell is membrane- 

 less. Nothing in the form of a membrane appears for some 

 time after the ovum becomes enclosed in a proper follicular 



' Kolessnikow (12, p. 400) makes a slip in saying that Gegenbaur refers 

 the origin of the vitelline spheres to the follicular epithelium. 



- I have seen nothing to favour the idea suggested by Gotte (7, p. 18), 

 that the first formed spherules move on toward the centre of the ovum, 

 and that their original places are occupied by newly formed spherules. 



