EARLY CHAXGES JN THE OVUM. 239 



vesicle, exactly as they are Hgured by Graber [114] (see PI. XIII., 

 Fig. 2). 



I conclude that the earlier cleavage of the polar cells occurs 

 in the Blow-fly during the passage of the germ through the 

 food-yelk. If a young morula is formed as I suggest, it is easy 

 to understand the difficulties which arise in the direct observa- 

 tion of the cleavage of the germ-yelk ; and I believe that the 

 morula is easily broken up into its constituent cellular elements, 

 which become scattered in the abundant food-yelk. This view 

 accounts for the segmenting nuclear spindles which have been 

 detected by the laborious researches of Blochmann [107] , and 

 others, lying scattered in the food-yelk. If the young morula 

 assumes the form of an open cylinder, as it does in Synapta, or 

 is perforated by intercellular openings, as it is in Echinus 

 (Selenka [101]), or remains an open cup, it is not difficult to 

 understand the manner in which the food-yelk passes into its 

 interior. 



The view that such is the origin of the blastoderm vesicle is 

 directly supported by Graber's figures [114, PI. VI., Figs. 60 to 

 67], and by his woodcut on p. 260, which I have reproduced in 

 PI. XII., Fig. 12. It is true that Graber regards these figures 

 as representing abnormal phenomena, but I regard them as 

 normal, and am at a loss to understand why he considers 

 them abnormal ; his only ground for the supposition appears 

 to be that they do not accord well with received views. 



Moreover, Metschnikoffs [98] description of the formation 

 of the blastoderm in the Scorpion exactly corresponds, so far as 

 the segmentation phenomena are concerned, with the descrip- 

 tion I have given above ; in these Arthropods at least yelk 

 segmentation is holoblastic, and the yelk grows after the morula 

 is formed. So also in Peripatus, if the observations of Kennel 

 are to be trusted — and they certainly appear to be far more 

 trustworthy than those made by his opponents — the yelk seg- 

 mentation in this group is holoblastic. 



In conclusion, I would observe that although the older 

 investigators held that the blastoderm in Insects appears 

 simultaneously over the whole surface of the yelk, more recent 



