100 PROCEEDINGS Of THE ANATOMICAL AND ANTHROPOLOGICAL 



bounds the umbilical orifice laterally. The amnion extends on to the 

 dorsal aspect of the body stalk, leaving its lateral and ventral aspects 

 uncovered a disposition similar to that in the embryo described by 

 Professor Thompson. 



MESODERMIC SOMITES AND NOTOCHORD. 



There are thirteen pairs of well-formed mesodermic somites, and 

 in addition to these there is an anterior pair incompletely formed. 



a n 



Fig. 1. Right half of a transverse section of the 

 embryo through the region of the eleventh meso- 

 dermic somite. 



EC., ectoderm ; XI. AI. S., mesodermic somite; /., segmental 

 tubule ; Co;,, ccelom ; d.Ao., dorsal aorta ; Sp.c., medul- 

 lary canal; Kn., entoderm; Ch., notochord. 



This most anterior and rudimentary pair of somites is situated just at 

 the level of the neck bend. The last pair of somites are well formed, 

 and situated at the level of the commencement of the rump bend. As 

 the ganglia are not developed, it is not possible to differentiate the 

 regions to which the somites belong. Each somite has a uniformly 

 thick wall three or four cells deep, enclosing a distinct cavity (myococl). 

 Towards the cavity many of the cells are undergoing mitotic division. 

 The more cranial-placed somites are triangular on cross-section, while 

 the more posterior ones are quadrangular (Fig. 1). The notochord 



