264 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



afterwards the female deposits her ova, already surrounded 

 by the characteristic capsule, which are then fertilized. 

 The ova are deposited in the capsules, which are probabl 

 secreted by the lining of the gonad. 



The ova before fertilisation measure about 'o mm. in 

 diameter, and are opaque owing to the numerous oily yolk 

 granules which they contain. The germinal vesicle is 

 well marked, and in it is a large nucleolus or germinal 

 spot. After fertilisation segmentation is complete and 

 regular, resulting in a blastula. A segmentation cavity 

 is already present in the 8 cell stage. The blastula is 

 covered with cilia by whose action the young embryo is 

 kept in constant rotation. Invagination of the blastula 

 then takes place, and results in the formation of a typical 

 gastrula. The differentiation between ectoderm and 

 endoderm cells is now apparent, the latter being con- 

 siderably larger. The endodermal invagination is directed 

 somewhat obliquely (PL IV., fig. 9), so that the future 

 alimentary canal lies entirely behind the blastopore, 

 enabling one to distinguish already at this stage the 

 anterior and posterior ends of the animal. 



The ectoderm of this stage does not directly become 

 the ectoderm of the larva, but the latter is established by 

 a series of remarkable changes. In two small areas on 

 either side the cells of the primary ectoderm of the gas- 

 trula divide lengthways forming palisade cells. These 

 areas of secondary ectoderm, the cephalic and ventral 



* The development of Lineus gesserensis has been studied more 

 especially by Desor, Barrois, M'Intosh, and Hubrecht. The account 

 given by the last-named is the only one based on modern methods, and 

 has been followed in this paper. 



