Chap. XIII.] THE EARTHWORM. 289 



the vasa deferentia at the period of congress. For though the 

 worm is hermaphrodite, the ova of one individual are fer- 

 tilised by the spermatozoa of a second. At such time the 

 spermatozoa are received by each worm from the other into 

 the spermathecse, and are there stored until the ova shall be 

 laid. 



It has been suggested that the oviducts, vasa deferentia, and 

 spermathecse are homologous with segmental organs; which 

 have in the generative segments been specially metamorphosed 

 in subservience to the process of reproduction. The evidence 

 for this view, however, involves careful comparison with other 

 allied forms of life, and is therefore outside the limits of this 

 work. 



The eggs are laid in special chitinous cases, probably secreted 

 by the cingulum. Certain so-called capsulogenous organs, modi- 

 fied from the setigerous capsules of the inner series in the 

 generative segments have, however, been assigned this func- 

 tion. Into each egg-case seminal fluid is poured from the 

 spermathecse. 



Development. Segmentation is complete and nearly regular. 

 It results in a flattened blastosphere forming a disc, composed 

 of an upper layer of smaller cells and a lower layer of larger 

 clearer cells. The disc becomes folded over by a process of 

 imagination, the larger and clearer cells becoming internal to 

 the others. Thus an elongated, two layered, more or less 

 ciliated gastrula is produced. The blastopore at first forms an 

 elongated slit at the folded edges, along the ventral surface; 

 but it gradually narrows to a pore which becomes the mouth. 

 The central or enteric cavity is lined by hypoblast cells, but at 

 the region of the mouth the epiblast becomes inflected so as 

 to line the cesophageal region. Thus there is no stomodseal 

 invagination. The anus and hind-gut are, however, formed by 

 a proctodseal invagination. 



The exact mode of origin of the mesoblast is a matter of 

 uncertainty. When formed it segments into quadrate masses 

 19 



