396 T. H. MORGAN AND UM#. TSUDA. 



Schultze maintains the same view for the brown frog, but 

 Roux, in a later publication_, denies this for this species. 



From Roux's conclusions, published in the ' Archiv fiir 

 Mikros. Anat./ vol. xxix, 1887, the following quotation is 

 taken : 



1. The unfertilised frog's egg has determined one main 

 axis of the median plane of the embryo. This results from 

 the bipolar arrangement of the yolk material, and corresponds 

 to the direction of the egg-axis passing from the black to the 

 white pole, i. e. to a ventro-dorsal direction of the real, a 

 cephalo-caudal direction of the virtual, embryo. 



2. From the innumerably different meridional planes which 

 can pass through the egg-axis, that one corresponds to the 

 median plane of the embryo that lies in the direction of the 

 copulation of the two pronuclei. 



3. The plane of copulation of the pronuclei is not in any pre- 

 determined meridian, but may be determined by localised 

 [artificial] fertilisation. 



4. The side of the egg where the sperm enters forms the 

 ventro- caudal side of the embryo; the opposite side corresponds 

 to the dorso- cephalic. 



In the body of the same paper Roux says that his experi- 

 ments show conclusively that there does not exist a latent bi- 

 lateral construction of the frog's egg. He further adds that 

 in this year he " was fortunate enough for the first time to 

 follow out with success the process of fertilisation in Rana 

 esculent a, and to observe that in this species a peculiar 

 typical change of position of the egg-axis takes place, i. e. the 

 black hemisphere sinks down 20° — 30°towards the 

 side of entrance of the spermatozoon." 



Hence the first line of cleavage that passes through the 

 upper pole of the egg and the line of the entering spermato- 

 zoon would also pass through the apex of the white crescent. 

 Logically no fault can be found with this ingenious explana- 

 tion ; but how explain Roux's earlier observation, that un- 

 fertilised eggs of Rana escul. also show the white cres- 

 cent ? Moreover the distribution of the pigment in the unfer- 



