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. 1 



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- 

 sym metrically, 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, Lemons s. I. Generation d. Vertebres. 

 Paris, 1879. In this work the author maintains very peculiar views on the nature and 

 function of the yolk nucleus, which do not appear to me well founded. 



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



FJG. 5. UNRIPE OVUM OK 

 TOXOPNEUSTES LiviDUs. (Copied 

 from Hertwig.) 



