DEVELOPMENT OF THE EARTHWORM. 197 



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 slightly unequal, 

 and exhibits considerable variation even within the limits of 

 a species. 



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 

 form all the muscles of the trunk, and which thus takes 

 origin from two primary mesoblasts. 



After five to six pairs of secondary mesoblasts have been 

 formed, the blastula begins to flatten, and to elongate, 

 becoming an oval disc. The cells of the lower surface 

 become clearer, and the hypoblast is thus defined. The cells 

 of the upper surface are smaller, and become very much 

 flattened ; they compose the epiblast. The mesoblasts lie 

 side by side near one end, forming two rows extending 

 forwards and downwards, but divergent, because of the 

 flattening of the blastula. The hypoblast now becomes 

 concave, and thus the blastopore arises, occupying the 

 whole of the lower surface. The sides close in and the 

 blastopore becomes a slit, which further closes from behind 

 forwards leaving only a small opening, the future mouth. 

 During these processes the cells at the anterior tip of the 

 blastopore, which will give rise to the praeoral lobe, undergo 

 no change, but the mesoblast has been active. 



As gastrulation proceeds, the mesoblast rows grow forwards 

 and upwards until they come near each other above the 



