424 Avorv E. LniTi])ert. 



Both Balfour and Locy report the appearance of nuclei on the 

 ventral surface of the egg at an earlier period than that at which they 

 appear on the dorsal surface. Kishinouye, however, with whose 

 observations my own accord in this respect, states that the nuclei 

 appear simultaneously, at first, over the entire surface of the egg, 

 and accumulate later, by division and migration, on the ventral 

 surface. 



Thus two distinct poles may be recognized in the egg of the spider ; 

 a vegetative pole, irregular in form and characterized by large, 

 irregularly projecting masses of yolk, its surface bearing relatively 

 few nuclei, those present being noticeably large — Fig. 2, ylk. — and 

 an animal pole where the yolk is compacted to form a comparatively 

 smooth, spherical surface, the cellular layer which covers its surface 

 being indicated by the presence of numerous, closely crowded nuclei. 



After the nuclei have migrated from the other portions of the egg 

 to the ventral region they continue to increase rapidly in number, 

 ultimately forming a cap of cells which covers this part of the egg. 

 This cap is called the blastodisc, Fig. 2, bid. In the meanwhile the 

 nuclei of those cells which have not become incorporated in the 

 blastodisc, but have remained on the vegetative pole of the egg, 

 increase considerably in size. 



Balfour states that the endoderm is derived from cells which do 

 not accompany the other cells in their migration to the surface, but 

 are left behind in the yolk mass. Locy, Morin, and Kishinouye, 

 however, failed to find any nuclei in the yolk in sections of either 

 the blastoderm or blastodisc stages. My earliest sections, which 

 were made with eggs whose development was somewhat in advance 

 of the blastoderm stage of the authors mentioned, bear out the latter 

 observations. Figs. 10, 11, and 12, ylk. In Figs. 15 and 16, the 

 yolk cells — y. c. — are migrating from the cells of the blastodisc, 

 from which they have been derived, into the yolk. This condition 

 appears not only to be true of this stage in the Araneina, but was 

 found by Patten (31) in NeopTiylax, and by Wheeler (41) in Blatta. 



A marked feature of the blastodisc is the occurrence of a depression 

 in about the middle of its area. (Figs. 2 and 3, hip. Fig. 10, hip ) 

 This depression is the center of an active proliferation of cells which 



