1892.] Experimental Embryology. 377 



caudal and tlu> opposite the dorso-cephalic side of the embryo. 

 An examination of these e^'s by sections shows that the 

 course of the sperm as indicated by the trail of dark pigment 

 left behind it is first a long radiating line from the black pole 

 near the white rim to a layer deep within the egg in which the 

 copulation of nuclei takes place, then a short course towards 

 the female nucleus. 



The author then points out that the cleavage in the plane 



both pronuclei, provided they are not intermixed in copula- 

 tion, half of each being readily moved into each of the result- 

 ing cells. 



When, however, the eggs are held inclined more than •Ju c - 

 30° the above rules do not hold good. Though even here the 

 first cleavage seems to coincide with the plane of copulation 

 of nuclei, yet these are often at right angles to the future 

 median plane of the frog, though they may coincide with it in 

 some cases. This is to be explained on the assumption that 

 gravity acting upon yolk and nuclei determines a rotation of 

 copulation direction. 



Again in these much inclined eggs the lowest side of the 

 black pole becomes the ventro-caudal part of the embryo, pos- 

 sibly because the formative yolk may accumulate t her- 1 and 

 influence the nuclear cleavage. 



The most interesting of Roux's contributions appeared a 

 year later. 9 In this he follows out the question of self-differ- 

 entiation of the embryo, seeking to determine the amount and 

 character of interaction of the part of an egg by destroying 

 or injuring definite cells during cleavage. The eggs of Rana 

 mmlenta in the two celled stage were at first simply stabbed 

 by a fine sharp needle, thrust into one cell, but as this did not 

 produce much injury the needle was heated and often moved 

 about inside the cell. Of such eggs about 20 per cent, devel- 

 oped only from the uninjured cell, others went to pieces or 

 else developed normally. It is to be noted also that at this 



9 V. Beitrage zur Entwicklungsmechanik des Embryo. Ueber die kunstliche 

 Hervorbringung halber Embryonen durch Zeistotung einer der beiden ersten 

 ler fehlenden Korper halite. 

 VircWi Arch 291. PI. 2-3. 



