MUSEUM OF COMPARATIVE ZOOLOGY. 251 



with another radial tube of its own hemisphere, and the two describe a par- 

 abolic curve convex toward the centre of the yolk. The canals contain a 

 liquid albuminous substance. In diameter they vary from 0.01'" to 0.0025'" 

 (22.0 /A to 5.6 /x). They may subserve, according to Reichert, the purpose of an 

 exchange of substance between the yolk and surrounding media, through a 

 process of diffusion. 



These canals continue to be traceable during the growth of the embryo, until 

 the yolk is reduced to a very small mass. 



Pfluger ('63, p. 79, Taf. V. Fig. 7) describes the " inner yolk " of the nearly 

 mature egg of the cat as being often quite sharply limited from the " outer yolk," 

 and still as presenting at the periphery a radial condition. One sees, he says, 

 that at different, though not numerous places, sharply limited processes (which, 

 I may parenthetically add, are very broad, and in no way recall the familiar 

 aster) reach out from the inner yolk to the zo7ia pellucida. " Man konnte dies," 

 he continues, " auch so auffassen, dass man sagte, es bestande im Eie um das 

 Keiniblaschen eine Hohle, Avelche durch radiar verlaufende sich allmahlich 

 verjiingende Canale mit der zona pellucida zu communiciren scheint." 



The observations of Oellacher ('72, pp. 6-14, Figs. 3-10) on trout eggs led 

 him to the conclusion that, while the egg is still in the ovary, the germinative 

 vesicle approaches its surface, and that the thick radially striate membrane of 

 the vesicle subsequently becomes evaginated through an opening at its most 

 superficial point. The contents of the vesicle thus become eliminated from the 

 egg, and the thick membrane is spread over a considerable portion of its sur- 

 face as a thin veil (Schleierchen) , which still continues to exhibit the radial 

 (uow palisade-like) structure. In the opinion of the observer the alternat- 

 ing dark and light striations are due to pore canals ; the lumen appears dark 

 (in a surface view as dark points), and the walls give rise to the light bands. 

 Biitschli inclines to the opinion that in this case the so-called membrane of 

 the vesicle is only a portion of the formative yolk which has become radially 

 sti'iate. But, to judge from Oellacher's Figure 6, the effect of this striating 

 inliuence does not extend into the yolk deeper than the rather sharply limited 

 substance called membrane, and its persistence throughout all the vicissitudes 

 of an evagination and ultimate elimination from the yolk bespeak for it a me- 

 chanical condition which is hardly paralleled in the molecular asters. 



In the eggs of certain snakes Eimer describes ('72'*, pp. 219, 220, 427, 428, Taf. 

 XI. Fig. 3) the germinative vesicle as surrounded by an inordinately thick, 

 very peculiar envelope (Hiille), "die aus sehr feinen Kornchen zusammen- 

 gebacken scheint und sich durch eine schone radiare Streifung auszeichnet." 

 In some places the rays are traceable into the surrounding yolk. In smaller 

 eggs one finds only a thin membrane ; in those that are larger this phenomenon 

 has disappeared, and there remains simply the thin membrane. Eimer ex- 

 plains this appearance as due to pore canals. " Die radiare Streifung ist wohl 

 als der Ausdruck von Poren, von feinen Rohrchen zu erklaren, welche die Hiille 

 durchsetzen." 



In addition to this circumvesicular striation Eimer ('72^^, p. 228, Taf. XL 



