130 DEVELOPMENT OF THE OVUM cuap. 



united with Clausilia ^;aj9t7/ar2S. 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 nemorcdis, H. asj^ersa and vermiculata, between 

 Stenogyra dccollata 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 Z. oMusata. and with Z. 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 

 occm- 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, only 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 reproductive systems. 

 The next, or trocliosphere (trocJiophora) stage, involves the forma- 



^ Journ. de Conchyl. iii. p. 107. 

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



