$96.] Embryology. 



Eggs that have not been shaken sometimes divide at < 



In both these unusual forms of cleavage the author finds that the 

 three or the four archoplasmic centres present in the egg take unequal 

 numbers of chromosomes. Thus in one case one centre was accom- 

 panied by 17, another by 14, another by 33 and the fourth by 

 chromosomes. 



That this inequality is greater in the fourfold than in the three-fold 

 division explains, the author suggests, the fact that fewer eggs develop 

 from the four- fold than from the three-fold cleavage. 



A third paper* gives a detailed account of the partial larvae obtained 

 when the eggs of Sphferechiuus are shaken into fragments. Very min- 

 ute gastrulse only b 1 ? part of the volume of a normal gastrula are thought 

 to come from isolated pieces with ?V to & the volume of the whole 

 egg- 

 It is found that the number of cells in such small blastula? is less 

 than the normal number and roughly proportional to the size of the 

 blastula. 



The size of the nuclei, and probably of the cells also, is less in the 

 small blastulse than in the normal ones. 



If one of first two cells of a cleaving egg be isolated it may form a 

 blastula with i the normal number of cells. One of the first four cells 

 gives a blastula with J the normal number, or with a little more than 

 i ; while one of the first eight cells when isolated produces a blastula 

 with more than I the normal number. 



Such blastulae will develop into gastrula}. 



A piece of the wall of a blastula when broken off* by shaking may 

 develop into a gastrula. 



The little blastuke formed from fragments of eggs tend to invaginate 

 as many cells as possible up to the normal number for a normal 



These remarkable numerical relations lead the author to suppose that 

 the reason why isolated cells of later stages in cleavage are not able to 

 develop by themselves lies not in any differentiation of nuclear sub- 

 stance but in the fact that such cells being themselves the results of a 

 series of cleavages cannot produce cells enough for the next stages of 

 development. 



Morgan and Driesch publish conjointly 5 their reinvestigation 

 of the remarkable halflarvae obtained by Chun. 



