40 PROCEEDINGS OF THE AMERICAN ACADEMY 



it reached a point on the opposite side of the equator. This ex- 

 tremely interesting fact reveals a fundamental agreement between the 

 frog and the fish in regard to the mode of formation of the embryo. 



By fixing the frog's ovuna in abnormal positions, at the time of 

 fecundation or soon after, Pfiuger has shown conclusively that the 

 direction of the first two cleavage-planes will invariably be vertical, 

 whatever be the position of the ovum ; and from this he concludes 

 that the line of intersection of these cleavage-planes coincides, under 

 normal conditions, with the axis of the ovum, for no other reason than 

 that this axis happens to represent the direction of gravitation. In 

 other words, gravitation determines the vertical direction of the first 

 cleavage-plane entirely independently of any pre-established axial re- 

 lations. Now such a conclusion stands in plain contradiction with our 

 general knowledge of cleavage ; and indeed is refuted, in our esti- 

 mation, by Pfiijger's own observations. Why is it that the first 

 cleavage-plane is invariably meridian in the ovum of the fish, not only 

 in those cases where the blastodisc lies at the upper or lower pole, but 

 also in those exceptional cases noticed by Ryder and others where it 

 lies at one side of the vitellus? If the cleavage-plane must follow 

 the direction of gravitation in the first instance, then why not in the 

 third ? What means this invariable order in the succession of the 

 early cleavage-planes? PHiiger should be the last man, after his ex- 

 periments, to declare that the order is the same because the outward 

 conditions are the same. If it is the purpose of the cleavage merely to 

 split up the germinal material into small pieces, and a matter of utter 

 indifference in what order this is accomplished, how does it happen 

 that in the teleostean ovum, according to our observations, <Ae ^rs< 

 cleavage-plane coincides with the median plane of the embryo ? And 

 is it merely a remarkable coincidence that, according to the concur- 

 rent testimony of both Pfliiger and Roux,*'' precisely the same relation 

 holds in (he ovum of the frog? Or shall we infer that because this 

 relation between the first cleavage-plane and the median plane of the 

 embryo may be reached by two modes of cleavage, " es ist ziemlich 

 gleichgiltig in welcher Reihenfolge die vorschreitende Zerkleinerung 

 sich vollzieht " ? 



If gravitation were the sole controlling and guiding agency in cleav- 

 age, its elfuct ought to be instantaneous, and it should be possible to 

 change the direction of a cleavage-plane already in progress. The 



*'■' Willielm Roux. Uebcr d. Zeit d. Bcstimmung d. Ilauptrichtuiigen d. 

 Frosclicmbryo. Leipzig, 18b3. 



