REPRODUCTION AND DEVELOPMENT 127 



exterior are formed from ectodermal tissue. The question of 

 whether the nephridioblast is ectodermal or mesodermal in origin 

 has not been finally settled. It may well be cut off very early 

 from ectomesoderm. 



Testes differentiate from the inner walls of the coelomic sacs 

 while these are still intact in such a way that each testis sac en- 

 closes a small part of the coelom. At a later stage the vasa efferentia 

 arise as evaginations of the testis sacs. They join together to form 

 the vas deferens of each side and these in turn unite with the 

 genital atrium which has differentiated from the somatopleure 

 anteriorly. A pair of ovarian sacs forms just behind the genital 

 atrium. They are first seen as a pair of gonoblasts cut off from the 

 walls of coelomic sacs and looking very like nephridioblasts. 

 Each gonoblast divides once and one of the cells thus formed 

 divides many times to form a group of cells which are organized 

 into a capsule surrounding the other cell. By elongation of the 

 capsule and subdivisions of the internal cell an ovarian sac is 

 formed with an ovarian strand inside it and a duct to the exterior. 



The segmentation of the mesoderm imposes a pattern of annula- 

 tion on the epidermis and circular muscles. At first there is only 

 one annulus per segment but soon a second annulus is cut off 

 from the posterior border of each original one. Then another is 

 cut off on the anterior border, giving the triannulate condition 

 characteristic of the majority of Glossiphoniidae. 



6. Embryology of a Gnathobdellid Leech 



The gnathobdellid leech which has been most often studied is 

 Erpohdella. The eggs are small and contain little yolk. The 

 cleavage follows the normal annelid pattern up to the formation 

 of the first quartette of micromeres. After this the blastomeres lA 

 and IB become passive and produce no more micromeres. IC 

 divides to form 2C and 2c and then it too becomes inactive. 

 2c passes into the blastocoel and only ID continues to divide. Its 

 next cleavage produces two cells of about equal size, 2d and 2D. 

 The latter cuts off two more micromeres, 3d and 4d and these join 

 2c in the blastocoel. At this stage the first quartette of micromeres 

 occupies the animal pole, macromere 4D is at the vegetative pole 

 and around the equator lA^ IB and 2C occupy left, anterior and 



