278 ELEMENTARY BIOLOGY. [CHAP. 



The conditions under which embryonic development is 

 passed through in the " Pond Snail" {Lymnceus stagnalis) are 

 much more favourable to observation than in the case 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 confined in a hand-glass or 

 table aquarium and well fed, the animals will very generally 

 deposit their eggs upon the vessel, and they will be found 

 in aggregates, firmly adherent to each other and to the sur- 

 face by means of the albuminous secretion aforementioned. 

 The important developmental changes are undergone while 

 still in this investment. 



The transverse diameter of the fertilized ovum is about 

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

 unequal, and the changes undergone during the early de- 

 velopmental 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. 



During segmentation there appear on the surface of the 

 dividing mass, some two or more minute protuberances, 

 which finally become constricted off and lost. A portion of 

 the nucleus of the cell whence each arises is carried away 

 with it. Various interpretations have been put upon these 

 polar bodies; they are of fairly general occurrence in the 

 animal kingdom, but further discussion concerning thera is 

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

 as the conditions of 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 



