CH. XI] EFFECT OF INJURING A BLASTOMERE H5 



off or absorbed. It has been shown by Roux that when a 

 blastomere has been pierced by a cold needle, there is a small 

 outflow of yolk, and the injured blastomere continues to divide 

 at the same rate as the uninjured cell. When the needle is 

 heated, the cleavage-process is delayed or prevented, while it 

 continues on the uninjured side ; but after a time the injured 

 blastomere may also begin to divide in an irregular way. 

 After two or three days one gets generally from such eggs 

 quite normal gastrulse and embryos, differing in little or no 

 respect from embryos from uninjured eggs. 



The nucleus of the uninjured blastomere may continue to 

 divide, although the protoplasm, owing to its injury, may not 

 be able to do so for some time. The nuclei may scatter them- 

 selves through the protoplasm (and yolk), and subsequently 

 take part in the division of this into cells. In extreme cases 

 Hertwig admits that when the needle is very hot, the whole 

 of the protoplasm of the blastomere may be killed, and also 

 the nucleus. Furthermore, it is possible that occasionally the 

 heat may radiate from the one blastomere into the other and 

 partially kill this other one also. If the last condition is 

 brought about, the development of the partially injured blasto- 

 mere may take place only very slowly, if at all. In most cases, 

 therefore, Hertwig believes a "reorganization" of the injured 

 cell takes place, and not a " revivification *' of the dead half. 

 In this reorganization, Hertwig thinks that the nucleus of the 

 injured cell itself plays the main part, while Roux believed 

 the process was brought about largely by an immigration of 

 cells (or nuclei) from the uninjured into the injured half. 

 Hertwig's conclusion here seems based rather on a priori 

 probability, while Roux's statements rest directly on his own 

 observations. Recently the same ground has been worked over 

 by Endres and Walter, whose results substantiate Roux in 

 every respect. 



Endres and Walter ('95) have obtained the typical half- 

 blastulse and half-gastrulae and half -embryos which Roux has 

 described. They deny that whole embryos develop from one of 

 the first two blastomeres, as Hertwig affirmed. Their figures 

 show in the most conclusive way that half-embryos do develop 



