18 



GENERAL HISTORY. 



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 unaymmetrically, 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 charac- 

 ters 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 membrane by the 

 action of reagents (Fol). The contents of 

 the germinal vesicle are for the most 

 part fluid, but may be more or less 

 granular. Their most characteristic 

 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 forms a nearly homogeneous body, with frequently one 

 or more vacuoles. It often occupies an eccentric position within the 

 germinal vesicle, and is usually rendered very conspicuous by its 

 high refrangibility. In many instances it has been shewn to be 

 capable of amoeboid movements (Hertwig, Eimer), and is moreover 

 more solid and more strongly tinged by colouring reagents than the 

 remaining constituents of the germinal vesicle. 



In many instances there is only one germinal spot, or else one main 

 spot and two or three accessory smaller spots. In other cases, e.g. 

 Osseous Fishes, Echiuaster fallax, Eucope polystyla, there are a 

 large number of nearly equal germinal spots which appear to 

 result from the division or endogenous proliferation of the original 



1 For details on the yolk nucleus vide Balbiaui, Lemons s. I. Generation d. Vertebres. 

 Paris, 1879. In this woik the author maintains very peculiar views on the uatme and 

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



* In the germinal vesicles of very young ova the retL-uluin is often absent. 



FIG. 5. UNRIPE OVUM OF 

 TOXOPNECSTES Livicus. (Copied 

 from Hertwig.) 



