140 WILLIAM BATESON. 



Figs. 26 — 34 are drawn from a series of sections through a 

 larva at this stage, of which fig. 13 is an external view. It 

 will be remembered that the body now consists of three 

 regions. (1) An anterior lobe which on the formation of 

 the mouth will be seen to be prseoral; (2) a narrow area 

 between two grooves, which will be afterwards spoken of as 

 the collar; and (3) the rest of the body, which will be 

 alluded to as the trunk ; this trunk portion is secondarily 

 divided into an anterior and posterior region by the transverse 

 band of cilia. 



The dorsal and ventral surfaces may now be distinguished in 

 section, as a thickening of the epiblast is already to be found 

 in the collar on one surface. This structure, which subse- 

 quently forms part of the central nervous system, marks the 

 middle dorsal line. The body is now elliptical in section, the 

 long axis of the ellipse being horizontal. 



The epiblast now consists of small elongated ceils, which 

 are arranged from two to three deep, bearing minute cilia on 

 their peripheral ends. This layer is of uniform thickness and 

 constitution over the whole body, with the exception of the 

 small linear area in the middle dorsal line of the collar^ men- 

 tioned above. In this region it is somewhat thicker, but no 

 difierentiation has occurred among its cells. Moreover, the 

 cells which carry the transverse band of cilia are slightly larger 

 and more columnar than the rest. 



The detailed structure of these epiblastic cells is shown in 

 fig. 31, a, E, They are small, dense cells, containing a consider- 

 able quantity of granules, which stain deeply with reagents. 

 The nucleus is proportionally large, and also contains granules. 



The mesoblast arises at this period of development. It is 

 formed directly by differentiations of cells belonging to the 

 archenteron. These differentiations occur in five regions. The 

 first comprises a median and primitively unpaired tract in the 

 anterior end, which forms the lining of the body cavity of the 

 praeoral lobe. Behind this anterior body cavity a pair of meso- 

 blastic differentiations occur, in the region of the collar, consti- 

 tuting lateral outgrowths of the archenteric walls, each con- 



