40 GEORGINA B. SPOONER 



micromeres and micropyle is only approximate even in the normal 

 eggs of Arbacia (fig. 9). Boveri, also, describes the first cleavage 

 as passing always through the micropyle pole. I found one 

 normal egg in which the first cleavage plane cut through at right 

 angles to the polar axis. 



The eggs which I isolated in 1908, not only show that the 

 normal relation of micromeres and gastrulation existed in the 

 centrifuged eggs, but also that the first cleavage plane does not 

 always pass through the micropyle pole as Boveri describes. 

 In studying these results, however, attention became centered 

 on the small, but nevertheless existent, number of eggs in which 

 the first cleavage was more nearly parallel to the stratification 

 than perpendicular. 



A small number of parallel cleavages had been reported in 

 all the earlier work in which the eggs were centrifuged before fer- 

 tilization. But so far as I know no observations had been recorded 

 on this point in eggs centrifuged after fertilization. My first 

 experiment, therefore, was to centrifuge two sets of eggs, one 

 before fertilization, the other forty-five minutes after fertiliza- 

 tion. In each case the watch glass colitaining the centrifuged 

 and fertilized eggs was placed under the microscope and a single 

 field was observed till the eggs had divided. In a small field of 

 the eggs, centrifuged before fertilization, only thirteen eggs 

 divided. In five of those eggs the cleavage was parallel to the 

 stratification. In a field containing over a hundred eggs, centri- 

 fuged after fertilization, the cleavage was visible in eighty-eight 

 eggs. They all divided within twenty minutes after removal from 

 the centrifuge and in every case the cleavage was perpendicular 

 to the stratification. This suggested that the irregularity of the 

 cleavage in the eggs centrifuged before fertilization might be due 

 to the fact that in those eggs the sperm enters after the centri- 

 fuging, and the sperm head and the aster are, therefore, not 

 directly affected by the force; while in eggs centrifuged just before 

 cleavage the whole karyokinetic figure is affected. 



With this possibility in mind I centrifuged a large number of 

 eggs before fertilization and killed some immediately after removal 

 from the machine and others at five minute intervals after fer- 



