REPRODUCTION AND DEVELOPMENT 121 



we shall see that in this feature as in many others the Hirudinea 

 resemble the oligochaetes. 



In terrestrial oligochaetes the formation of the germinal bands 

 proceeds much as in Tuhifex but the formation of quartettes of 

 micromeres and of the endoderm is frequently modified by the 

 failure of two of the four original blastomeres to take any part in 

 development. In Bimastus for instance blastomeres A and B form 

 no micromeres. B divides once, A not at all and these cells lie at 

 the animal pole for a considerable period before finally degenerating. 



Against this background of annelid embryology we may turn to 

 the embryology of leeches. In general the Glossiphoniidae have 

 the least modified type of spiral cleavage and have relatively large 

 amounts of yolk, while the eggs of other families show greater 

 deviations from the typical pattern of spiral cleavage and have less 

 yolk, relying more on the supply of albuminous fluid contained 

 within the cocoon. 



5. Embryology of a Glossiphoniid Leech 



The egg of a glossiphoniid is about 0-5-1 -0 mm in diameter and 

 is often coloured green or yellowish by the yolk. First and second 

 polar bodies are cut oflF in the first hour or two after the eggs have 



Fig. 72. Egg of Theromyzon tessulatwn at 2-ceIl stage showing 

 pole plasm. From Dawydoff, 1959, after Schmidt. 



been laid and soon afterwards two masses of clear cytoplasm, the 

 pole plasms, become differentiated at the animal and vegetable 

 poles. The animal pole plasm has a ring formation and the 

 vegetative pole plasm is disc-shaped. The first cleavage is into 



