2/8 ELEMENTARY BIOLOGY. [CHAP. 



The conditions of development of the " Pond Snail" 

 (Lymnaus stagtialis] are much more favourable for observa- 

 tion than those of the "Common Snail," and they are 

 accordingly here dealt with. 



The " Pond Snail" is a sluggish carnivorous animal which 

 may be found in abundance during the summer months in 

 ponds and stagnant waters ; it is moreover an indispensable 

 acquisition to the aquarium. If well fed, the animals will 

 deposit their eggs upon the vessel, in aggregates, imbedded 

 in the albuminous secretion aforementioned. The more 

 important developmental changes are undergone within this 

 investment. 



The transverse diameter of the fertilized ovum is about 

 the ^-jfoth of an inch. The segmentation is holoblastic and 

 unequal, and the changes undergone during the early 

 developmental period are substantially such as have been 

 already described for the Earthworm (cf. p. 247), resulting 

 in the formation of a simple two-layered gastrula. 



Prior to segmentation there appear on the surface of the 

 egg some two or more minute protuberances, which finally 

 become constricted off and lost 1 . A portion of the nucleus 

 of the egg cell is carried away with each body thus formed. 

 Various interpretations have been put upon these polar- 

 bodies; they are cellular in nature and of general occurrence in 

 the animal kingdom, but further discussion concerning them 

 is beyond the scope of this work. They are alluded to here, 

 as the conditions for observation are exceedingly favourable. 



After the gastrula phase is passed, the embryo assumes a 

 somewhat spherical shape, during which period the mouth 

 is formed as a median involution of the epidermis. There 

 now appears on the surface, immediately in front of the 



1 In this animal they usually remain adherent to the fertilized egg 

 until segmention is far advanced. 



