CHAP. XL.] DOTATION OF THE YOLK. 571 



that the ovum reaches the uterus, except in the case of the rabbit, 

 where Bischoff has observed that they are lost as soon as it enters 

 the Fallopian tube. 



Soon after the escape of the ovum from the follicle, and, in 

 some instances, even before this, the germinal vesicle disappears. 

 According to the observations of some observers, amongst whom 

 may be mentioned Barry and Wagner, the germinal vesicle is the 

 seat of cell-formation.* The nature of the earliest changes, how- 

 ever, are very obscure. Eventually, two cells result, which are 

 destined to undergo subsequent division and subdivision. 



As the ovum passes along the Fallopian tube, it increases some- 

 what in size : the yolk becomes of firmer consistence, and shrinks 

 a little, so as to leave a cavity between it and the zona pellucida, 

 which is occupied with a clear fluid. The original vitelline mem- 

 brane itself, according to Dr. Barry, disappears by liquefaction. 

 The external investing membrane seems to be derived from the 

 mucous membrane of the Fallopian tube, and it is this which at 

 length becomes developed into the chorion. 



Dr. Bansom describes the changes in the yolk immediately resulting from 

 impregnation, in the stickleback's egg, as follows : " About fifteen or twenty 

 minutes after impregnation, a remarkable and more vivid contraction begins, 

 causing the yolk to pass through a series of regularly recurring forms. The 

 contraction begins on one side, near the equator, and soon forms a circular 

 constriction, which gives the yolk the figure of a dumb bell, the longer 

 axis of which is the polar axis of the egg. The constriction travels towards 

 the germinal pole, and next produces a flask-shaped figure ; this is at length 

 lost by the constriction passing on, and the round form is regained in about 

 a minute. This wave re-appears, and travels forward again, without any 

 distinct period of rest ; and I have seen these movements continue for forty- 

 five minutes, though towards the latter part of this period they are less 

 distinct and more limited in extent. The germinal mass itself, during these 

 contractions, which strongly resemble the peristaltic movements of the intes- 

 tine, undergoes changes in form, and has increased in bulk and distinctness. 

 These movements are unaffected by weak galvanic currents. Cleavage begins 

 in about two hours after impregnation : no embryonic cell was observed 

 before it began, nor in any of the cleavage masses." 



Rotation of the Yolk. At a very early period after impregnation, 

 Bischoff has observed, in the ovum of the rabbit, guinea-pig, and 

 some other animals, the very interesting phenomenon of the rota- 

 tion of the yolk. The movements were of a regular rotatory 



" 



* J. Miiller has shewn that in EntocUoncha mirdbilis (one of the mollusca), 

 the germinal vesicle does not disappear, but forms the origin of the em- 

 bryonic cell in the centre of the yolk. 



p P 2 



