EARTHWORM. 



207 



Development. Many 

 cocoons are made about the same 

 time, and each contains numerous 

 ova, and also packets of sperms, 

 so that fertilisation takes place 

 outside the body. These cocoons 

 are buried in the earth a few 

 inches below the surface. They 

 measure about a quarter of an inch 

 in length. 



The favourite time for egg- 

 laying is during the spring and 

 summer, though it may be con- 

 tinued throughout the whole year. 

 The earthworm of the dungheap 

 (L. fcet^d^^s'} makes this a habit, 

 induced probably by the warmth 

 of its environment. 



Of the many ova in the cocoon 

 of L. terrestris, only one comes to 

 maturity, while in L. fcctidus a 

 few, and in L. communis two 

 may do so. But in the last 

 species the two embryos are often 

 twins formed from one ovum, 

 separation taking place at the 

 gastrula stage. 



The whole process of growth, 

 until leaving the egg, lasts from 

 two to three weeks, the time vary- 

 ing, however, with the tempera- 

 ture. 



The ovum is surrounded by a 

 vitelline membrane, and is laden 

 with yolk granules. Segmentation 



FIG. 99. Stages in the develop- 

 ment of earthworm. After 

 Wilson. 



1. Two-celled stage; p.c., polar bodies. 



2. Blastula ; M., a primary mesoblast. 



3. Gastrula stage; EC., ectoderm or 



epiblast ; En., endoderm or hypo- 

 blast, in process of being covered 

 by the small ectoderm cells. Note 

 the widely open blastopore ; J/.. 

 mesoblast cells. 



4. Longitudinal section in late gastrula 



stage, showing germ - bands ; ec., 

 ectoderm ; en., endoderm ; M., 

 mouth ; st., stomodaeum ; ;., 

 primary mesoblasts ; Nl>., neuro- 

 blasts ; nc., nerve -cord ; IV., ne- 

 phridioblasts ; < is., mesoderm 

 bands; /r., incipient nephridia. 



N 



