220 SEGMENTED WORMS OR ANNELIDA. 



the narrow canal of the muscular penis, pass out on the 

 middle ventral line between rings thirty and thirty-one, and 

 are transferred in copulation to the female duct of another 

 leech. 



The female organs are more compact. The two small 

 tubular and coiled ovaries are enclosed in a spherical vesicle, 

 the walls of which are continued as two oviducts which unite 

 together in a convoluted common duct. This is surrounded 

 by a mass of glandular cells, which exude a glairy fluid into 

 the duct. Finally, the duct opens into a relatively large 

 muscular sac the "uterus," which opens through a sphincter 

 muscle on the middle ventral line between rings thirty-five 

 and thirty-six. 



The favourite breeding time is in spring Two leeches 

 fertilise one another, uniting in reverse positions so that the 

 penis of each enters the uterus of the other. Spermatophores 

 are passed from one to the other, and the contained sperms 

 may remain for a long time within the uterus, or, liberated 

 from their packets, may work their way up the female duct, 

 meeting the eggs at some point, or reaching them even in 

 the ovaries. 



The development has been most carefully worked out for 

 the little leech Clepsine, and we shall follow it there. 



Development of Clepsine. 



The eggs are laid in water, and surrounded by a cocoon ; they are large, 

 and contain much food yolk. 



Cleavage is complete but unequal. At the four cell stage, there are 

 three sub-equal smaller cells and one larger posterior cell, which marks 

 the future hind end of the body. From each of these cells a small cell 

 is cut off, and in this way four macromeres and four micromeres are 

 produced. The number of micromeres is continually increased by the 

 splitting off of cells from the macromeres, so that a disc of small cells is 

 formed. Except for this continued splitting off of small cells, three 

 of the four macromeres remain passive for a considerable period ; they 

 contain most of the food yolk, and serve as reservoirs of nutriment. The 

 other, or posterior, macromere divides into two cells of unequal size, the 

 larger speedily again divides into two primitive mesoblasts, the smaller 

 divides into eight symmetrically arranged cells, the neuro-nephroblasts. 

 At this time free nuclei appear in the other three macromeres (ento- 

 blasts) without any corresponding process of cell division, these surround 

 themselves with protoplasm, and form the endoderm cells lining the gut. 

 The disc of small cells is now spreading over the surface of the entoblasts, 

 over the neuro-nephroblasts, and over the primitive mesoblasts, which 



