DEVELOPMENT OF THE EARTHWORM. 173 



They make many cocoons 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 continued throughout the whole 

 year. The earthworm of the dung heap (Z. fmtidus) makes 

 this a habit, induced probably by the warmth of its habitat. 



Of the many ova of the earthworm L. terresfris, only 

 one comes to maturity, while of L.foeiidus a few, and of 

 Z. 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 varying however with the 

 temperature. 



The ovum is surrounded by a vitelline membrane, and is 

 laden with yolk granules. It seems that several polar cells are 

 formed, probably by division of the two primary ones separated 

 from the ovum. Segmentation is rather unequal, and periods 

 of rest alternate with periods of rapid division. Even the 

 first cleavage divides the egg-cell unequally, the second 

 divides the smaller part equally, and seems to cut off a 

 small cell from the larger part. A resting stage now occurs, 

 after which the division becomes more and more irregular, 

 individual ova also presenting special peculiarities. During 

 division an opening called a cleavage pore often occurs 

 between the cells. 



In about twenty-four hours, a nearly spherical, one-layered 

 blastosphere or blastula is formed. It consists of only about 

 thirteen cells. During the next twenty-four hours the cells 

 increase in number rapidly, but the blastula remains one- 

 layered. Two cells lying together do not take part in this 

 division; they are rather larger than the rest, and their 

 inner ends project into the cavity and are soon cut off. 

 Gradually these large cells begin to sink in, giving rise to 

 more daughter cells, and at last are quite included in the 

 cavity. Thus there arise two parallel rows of cells within 

 the blastula, and these define the longitudinal axis of the 

 embryo. This is the beginning of the mesoblast which will 



