360 L. DONCASTER. 



into such close contact that observations in the living state 

 are more difficulty the sections become more necessary. At 

 a stage when the curvature of the embryo is still not much 

 marked^ when the mouth invagination and the head cavities 

 have recently been formed, a transverse section through the 

 head (PL 19, fig. 9) shows the epiblast thickest dorsally and 

 laterally, and quite thin on the ventral side. The mouth 

 invagination is seen opening ventrally, and on each side of 

 it are the two masses of cephalic mesoderm in which the 

 cavity is already obliterated, but the nuclei can be seen 

 arranged in a double row, showing how the cavity originally 

 la}' between them. These masses of cephalic mesoderm are 

 dorsally in contact with one another, but ventrally they are 

 separated by the mouth invagination, so that their shape in 

 transverse section resembles that of a horseshoe. When 

 traced back into the neck it is seen that they overlap the 

 front ends of the trunk mesoderm, which lies nearer the 

 middle line, so that in some sections parts of both can be 

 seen at once, and hind ends of the head mesoderm masses 

 lying at the outer sides of those of the trunk. 



The ectoderm of the head, besides its anterior dorsal 

 thickening which gives rise to the cerebral ganglion, is also 

 thickened at the sides in the month region, where it forms a 

 layer, two or three cells deep laterall}^, while it is only one 

 cell thick on the dorsal surface. This thickening is the 

 rudiment of the hood ("Kappe"), the formation of which 

 will be more fully described later. 



The alimentary canal in the head fills up a considerable 

 space; it is roughly oval in transverse section just behind 

 the mouth, and consists of a layer of well-defined cells which 

 in the earlier embryos enclose a distinct space, but this 

 disappears later. The alimentary canal in the head is 

 entirely derived from the ectodermal invagination by which 

 the month is formed, and the true endodermal part only 

 begins in the neck region. As the embryonic curvature 

 proceeds, the mouth lies more on the ventral (inner) side, 

 and becomes elongated, so as to form a somewhat slit- like 



