130 DEVELOPMENT OF THE OVUM chap. 



united with Clausilia papillaris. No offspring seem to have 

 resulted from what the professor calls 'this innocent error,' for 

 the wall was carefully scrutinised for a long time, and no hybrid 

 forms were ever detected. 



The same observer noticed, in the Luxembourg garden at 

 Paris, and M. Gassies has noticed ^ at various occasions, union 

 between Helix aspersa and nemoralis^ H. as2?ersa and vermiculata^ 

 between Stenogyra decollata and a Helix (sp. not mentioned), H. 

 variabilis and pisana^ H. nemoralis and hortensis. In the two 

 latter cases a hybrid progeny was the result. It has been noticed 

 that these unions generally took place when the air was in a 

 very electric condition, and rain had fallen, or was about to fall, 

 abundantly. 



Of marine species Littorina rudis has been noticed ^ in union 

 both with L. ohtusata and with L. littorea^ but no definite facts 

 are known as to the result of such unions. 



Self -impregnation (see p. 44). 



Development of the Fertilised Ovum. — The first stages in 

 the development of the Mollusca are identical with those which 

 occur in other classes of animals. The fertilised ovum consists 

 of a vitellus or yolk, which is surrounded with albumen, and is 

 either contained in a separate capsule, or else several, sometimes 

 many, ova are found in the same capsule, onl}" a small proportion 

 of which ultimately develop. The germinal vesicle, which is 

 situated at one side of the vitellus, undergoes unequal segmenta- 

 tion, the result of which is usually the formation of a layer of 

 small ectoderm cells overlying a few much larger cells which 

 contain nearly the whole of the yolk. The large cells are then 

 invaginated, or are simply covered by the growth of the ectoderm 

 cells. The result in either case is the formation of an area, the 

 blastopore, where the inner cells are not covered by the ectoderm. 

 The blastopore gradually narrows to a circular opening, which, 

 in the great majority of cases, eventually becomes the mouth. 

 The usual differentiation of germinal layers takes place, the 

 epiblast eventually giving rise to the epidermis, nervous system, 

 and special sense organs, the hypoblast to the liver and to the 

 middle region of the alimentary tract, the mesoblast to the 

 muscles, the body cavity, the vascular, the excretory and repro- 



1 Journ. de Conchyl. iii. p. 107. 



2 Jeffreys, Brit. Conch, iii. p. 359 ; Sauvage, Journ. de Conchyl. xxi. p. 122. 



