266 SOME RECENT 



effected imperfect separation of the two blastomeres, 

 without rupturing the egg membrane. From these 

 eggs either twins or double embryos were produced, 

 a result which suggests that the twinning which 

 occurs so commonly in Lumbricus trapezotdes may 

 not after all be a result of double fertilisation as 

 was supposed by Kleinenberg. 



The most recent and the most complete of the 

 experiments on the results of shaking apart the 

 blastomeres of segmenting eggs are those performed 

 by Wilson on Amphioxus eggs in 1892. The 

 eggs of Amphioxus are very minute, about O'l mm. 

 in diameter ; and their early stages of development 

 are extremely simple. The egg divides by a 

 vertical cleft into two equal blastomeres : by a 

 second vertical cleft, at right angles to the first, 

 each of the two blastomeres is bisected, and four 

 blastomeres of equal size result. The third cleft 

 is a horizontal one, and is rather nearer the upper 

 pole than the lower : by it each of the four 

 blastomeres is divided into an upper and rather 

 smaller cell, and a lower and rather larger one. 

 In the later stages the number of cells is rapidly 

 increased ; a blastula, and then by invagination 

 a gastrula is formed, and the several organs of the 

 embryo quickly appear. In the early stages the 

 blastomeres hang together very loosely, and are 

 easily separated by shaking. Commencing with 

 the stage at which two blastomeres are present, 

 Wilson isolated them by shaking, and found that 

 each developed as though it were a complete egg, 

 segmenting in the normal fashion, giving rise to 



