CLEAVING OF THE YOLK. 65 



As the ovum approaches the middle of the Fallopian tube, it begins to 

 receive a new investment, consisting of a layer of transparent albuminous 

 or glutinous substance, which forms upon the exterior of the zona pellucida. 

 It is at first exceedingly fine, and, owing to this and to its transparency, it is 

 not easily recognised; but as the ovum reaches the lower part of the Fallopian 

 tube, this new investment acquires considerable thickness, and shortly 

 begins to assume the characters of the chorion, into which it, together with 

 the zona pellucida, is subsequently converted.* At this part of its transit 

 along the Fallopian tube, the ovum remains still unchanged in structure, 

 and no alteration, beyond increased thickness, is perceived in the zona 

 pellucida. A remarkable phenomenon has, however, been noticed by 

 Bischoff, about this time, namely, the rotation of the yolk within the zona 

 pellucida a phenomenon produced, he says, by the action of vibratile 

 cilia, situated upon the surface of the yolk. This curious observation has 

 been described in a note at page 1564 of MUller's Physiology.f 



The changes which the mammalian ovum undergoes in its passage 

 through the second half of the Fallopian tube, consist in the further forma- 

 tion of the chorion, and in the peculiar process of cleaving of the yolk, 

 which will now be discussed. The development of the chorion, will be 

 considered at a future page. 



Division and subdivision of the yolk. 



This process has been long known to occur in the amphibia and fishes 

 and some invertebrate animals. And observations of Dr. Barry, relative 

 to the rabbits' ovum, seemed to shew that a similar change occurs in the 

 ovum of mammiferous animals. This is now known with certainty to be 

 the case. The exact nature of the process is, however, still involved in 

 doubt, and very different opinions respecting it are entertained by different 

 physiologists. 



The phenomena are observed with more difficulty in the higher than in 

 the lower animals. Hence, it seems desirable to detail those which have 

 been recently investigated under favourable circumstances, and with great 

 ability, by Professor Kolliker \ and Dr. H. Bagge, in many of the In- 



* This deposit of albuminous matter around the ovum, first described by Mr. Wharton 

 Jones, appears to have been yet observed only in the rabbit ; no such deposit takes place 

 around the ovum of the bitch (Bischoff, 1. c. page 46), in which animal the chorion is formed 

 from the zona pellucida alone. 



t The rabbit appears to be the only mammalian animal in whose ova this rotation of the 

 entire mass of the yolk, previous to cleavage, has been observed. Bischoff has never detected 

 it in the bitch's ovum, but the Fallopian tube in these animals is so thick and opaque, that 

 the phenomenon might occur without being perceived (Entw. des Hunde-eies, p. 46). 



Entwickelungs-geschichte wirbellb'ser Thiere, Muller's Archiv. 1843, p. 68; and 

 Entwickl. der Cephalopoden, Zurich, 1844. 



De Evolut. Strongyli auric, et Ascarid. acum. Vivip. Diss. Inaug. Erlangae, 1841. 



F 



