296 NATURAL SCIENCE. April. 



other words, the orientation of the ovum is regulated by internal 

 conditions. Born reached the same conclusion by another method. 



Having settled this point, Roux attacked the question whether 

 all parts of the ovum were necessary to normal development. After 

 two or more divisions, certain cells of the segmenting egg were pricked 

 by a needle, so that some of the substance was lost. Thereafter, 

 the development was watched until the embryo became differentiated. 

 The result was that a loss of even one-sixth of the substance of the 

 ovum produced only circumscribed local defects or disturbances. To 

 develop fairly well, an ovum need not be intact. 



Then Roux proceeded to exclude from development one of the 

 two first segmentation cells. This was done by puncturing one cell 

 with a hot needle. The result was that a typical half-morula, or half- 

 gastrula, or half-embryo developed. Thus, there might be half 

 cerebral vesicles, one auditory vesicle, a half-gut, a single row of 

 proto vertebrae, and a notochord of half the normal thickness. Some- 

 times, however, these half-embryos were slightly abnormal. Thus it 

 was proved that one of the first two segmentation cells may form half 

 an embryo ; that it has not only the requisite vital material, but the 

 requisite power ; and, that it can develop apart from its neighbour. 

 It is a likely inference that in normal development, a similar indepen- 

 dence does to a certain degree obtain. 



What is true when the first segmentation-plane corresponds to 

 the future median one, is also true when the first plane corresponds to 

 the future transversal, as is sometimes the case in the segmentation of 

 frog ova. In other words, Roux was able to produce not only a right 

 and left half-embryo, but also an anterior and a posterior half-embryo. 

 By destroying one of the first four segmentation-cells, he was able to 

 rear three-quarter embryos ; by destroying three of the four, he got 

 quarter-ga^trulae. Indeed, he goes the length of maintaining that the 

 gastrulation of the frog ovum is normally a kind of mosaic work, 

 formed in at least four vertical, independently-developing pieces. 



Very remarkable is the process by which the half-embryo may 

 form a whole by vitalising the injured half-egg which has been lying 

 passive while the uninjured half has been developing. To this 

 process Roux applies the term post-generation. Nuclei and, perhaps, 

 also portions of protoplasm migrate from the uninjured side into the 

 passive unsegmented mass ; a remarkable kind of cell-division is 

 set up ; and gradually, in a peculiar manner which Roux describes, 

 the missing half is post-generated. Quite lately, however, Roux has 

 been able to rear an entire frog embryo from half an egg without any 

 co-operation on the part of the other injured half. 



III. Professor C. Chun observed in 1877 that when the two first 

 segmentation-cells of a Ctenophore ovum were shaken apart, each 

 formed a half-larva, with four ciliated ridges, four meridional vessels, 

 and one tentacle. The half-larvae actually became sexual, and by a 

 process of budding, the half awanting was formed. This observation, 



