22 



GENERAL HISTORY. 



nucleus, which is very possibly connected with the formation of 

 the food-yolk. It is found in many Arachnida, Myriapoda, 

 Amphibia, etc. ' 



More important for the subsequent development than the 

 variation in the character of the food-yolk is its amount and 

 distribution. In a large number of forms it is distributed un- 

 symmetrically, the yolk being especially concentrated at one 

 pole of the ovum, the germinal vesicle, surrounded by a special 

 layer of protoplasm comparatively free from food-yolk, being 

 placed at the opposite pole. In the Arthropoda it has in most 

 instances a symmetrical distribution. Further details on this 

 subject are given in connection with the segmentation ; the 

 character of which is greatly influenced by the distribution of 

 food-yolk. 



The body of the ovum is usually spherical, but during a 

 period in its development it not unfrequently exhibits a very 

 irregular amoeboid form, e.g. Hydra (fig. 4), Halisarca. 



The germinal vesicle. The 

 germinal vesicle exhibits all the 

 essential characters of a nucleus. 

 It has a more or less spherical 

 shape, and is enveloped by a distinct 

 membrane which seems, however, 

 in the living state to be very often 

 of a viscous semi-fluid nature and 

 only to be hardened into a mem- 

 brane by the action of reagents 

 (Fol). The contents of the germi- 

 nal vesicle are for the most part 

 fluid, but may be more or less 

 granular. Their most characteris- 

 tic components are, however, a protoplasmic network and the 

 germinal spots 2 . The protoplasmic network stretches from the 

 germinal spots to the investing membrane, but is especially 

 concentrated round the former. (Fig. 5.) The germinal spot 



1 For details on the yolk nucleus vide Balbiani, Lefons s. 1. Gyration d. Vertfbrh. 

 I'aris, 1879. In this work the author maintains very peculiar views on the nature and 

 fu;:ction of the yolk nucleus, which do not appear to me well founded. 



* In the germinal vesicles of very young ova the reticulum is often absent. 



FIG. 5. UNRIPE OVUM OK 

 TOXOPNEUSTES LiviDUS. (Copied 

 from Hertwig.) 



