Katharine Foot and E. C. Strobell 203 



spindle are first seen at opposite poles of the germinal vesicle, indicating 

 that the}' arise independently and not by division of a primary centriole. 

 This stage is shown in Photos. 81, 82, 84 and 86, Plate V. The eggs 

 of Photos. 81 and 82 are greatly marred by poor fixation, chromo-acetic 

 being as harmful to cytoplasm as it is to nuclear structure (see p. 215). 

 The centriole of each aster is so small that only the presence of the rays 

 justifies our interpreting the central microsome as the centriole. In Photo. 

 81, Plate V, the centriole is in contact with the membrane of the germinal 

 vesicle, and in Photo. 82, Plate V, only slightly removed from it. The two 

 asters are at opposite sides of the germinal vesicle, being separated by 

 fifteen sections, the entire germinal vesicle being cut into ninteen sections. 

 In all these sections the membrane of the germinal vesicle is intact and at 

 the points where the rays of the asters focus the membrane slightly 

 protrudes." 



It might be claimed that the theory of the nuclear origin of the cen- 

 triole is supported by the presence of these two centrioles almost within 

 the germinal vesicle, but they certainly do not support the particular 

 phase of the theory that holds the nucleolus responsible for the centriole, 

 for in this egg the nucleolus is still intact and is found two sections 

 removed from one aster and thirteen sections from the other. The 

 injurious action of chromo-acetic on the nucleoplasm of the germinal 

 vesicle does not always affect its membrane, although it breaks the con- 

 nection between the two, leaving the membrane in contact with the sur- 

 rounding cytoplasm, this being favorable for the identification of the 

 earliest appearance of the centriole. On the contrary other photographs 

 show that many fixatives favorable for the study of nuclear constituents 

 are of no value for studying the early appearance of the centrioles, the 

 cytoplasm being torn away from the germinal vesicle, destroying all trace 

 of the centrioles and asters (Photos. 22 to 25, 29 and 30, Plate II, 4G to 

 ■49, 51 and 52, 59 to 63, and 65, Plate III). In a few cases we have 

 found eggs killed in these same fixatives showing an equal shrinkage of 

 cytoplasm and nucleus leaving the membrane in continuity with cytoplasm 

 and nuclear reticulum, but the exceptions are very rare and have not been 

 at the stage to throw any light on the origin of the centrioles. Platino- 

 osmic, as stated above (p. 200), appears to be the least injurious for all 

 constituents, and we hope its further use will enable us to collect more 



'^ The blurred effect of the part of the membrane seen in the photographs is 

 due to its being on a different plane from the centrioles. The two sections 

 shown in Photos. 81 and 82 are cut so close to the periphery of the germinal 

 vesicle that part of its membrane is seen en face. In the sections beyond 

 those photographed, the membrane is the only part of the germinal vesicle left. 



