226 WILLIAM BATESON. 



third of the collar this thickening is much more marked, and 

 is formed by a cord of columnar cells whose characters differ 

 from those of the rest of the epiblast (fig. 38, n. s.). These 

 cells are rather larger and somewhat pyramidal in section, their 

 bases forming the inner border of the epiblast in this region. 

 Their apices converge towards the centre of the cord. By a 

 continuation of this process of convergence this portion of the 

 epiblast in the posterior third of the collar segregates itself 

 from the skiu, forming an apparently solid rod of cells imme- 

 diately below the epiblast but detached from it in the dorsal 

 middle line of the collar (fig. 39, n. c.) This separation from 

 the skin is extended backwards and forwards along the whole 

 length of the collar, but is never completed at either end of it, 

 where the continuity persists throughout life. 



As has been stated this nerve cord is at this stage apparently 

 solid ; but as may be seen upon examination of the same struc- 

 ture in the adult it eventually possesses a distinct lumen for a 

 great part of its course. 



[In fig. 39 it will be observed that a transverse section cuts 

 both the middle and posterior body cavities. This is due to a 

 forward growth of the posterior body cavities on the ventral 

 side of the middle pair. As a result of this growth the septa 

 dividing the two cavities come to lie obliquely, instead of being 

 in a transverse place.] 



As it is not proposed on the present occasion to proceed 

 beyond this point in the account of the development I will 

 now briefly recapitulate the chief facts in the history of the 

 larva. 



Recapitulation. — The eggs are elliptical and opaque, being 

 fertilised outside the body. After impregnation they divide 

 into two, the subsequent segmentation being probably regular 

 and complete. Segmentation results in the formation of a 

 hollow blastosphsere, enclosing an empty segmentation cavity. 

 One side of this blastosphsere is next invaginated to form the 

 hypoblast, thereby con-stituting a simple hemispherical gastrula. 

 The blastopore closes completely ; the point of closure being 

 placed at the middle dorsal edge of the posterior surface. At 



