HUMAN EMBRYO OF THIRTEEN SOMITES 327 



of the cloaca is fused with the body ectoderm and forms a thick 

 cloacal membrane. At the most caudal part of the cloaca there 

 is a thickening of the entoderm together with a slight evagina- 

 tion which is suggestive of a post anal gut. 



NOTOCHORD 



The notochord is about in the same stage of development as 

 the one described in a 2.5 mm. embryo by Kollmann ('90). The 

 notochord is intimately connected with the gut entoderm through- 

 out its length with the exception of the caudal end. The caudal 

 end, or tail bud, is cut off from the entoderm and lies imbedded 

 in the mesoderm between the neural ectoderm and the gut tube. 

 There is no distinct notochordal canal as described by Mall ('91), 

 Eterriod ('99) and Grosser ('13). In places the cells of the noto- 

 chord are vacuolated and apparently in a stage of developing 

 a canal. The relationship of the notochord to the gut entoderm 

 is a very intimate one. In the region of the mid-gut the noto- 

 chord is composed of but a single layer of cells which appear to 

 be a modified part of the gut entoderm. Where the notochord 

 is composed of more than a single layer of cells the basal layer 

 is directly continuous with the single layer of cells forming the 

 gut entoderm. It is impossible to give any other interpretation 

 than that the notochord is developed from the gut entoderm. 

 In places the cells of the notochord are arranged in two lateral 

 masses giving the appearance of bilateral symmetr}^ This con- 

 dition is undoubtedly accounted for by the arched nature of the 

 original notochordal plate. In the subsequent proliferation of 

 cells they would grow laterally and unless there were an espe- 

 cially active growth of cells in the central part a gap would nat- 

 urally intervene between the two lateral groups of cells. At the 

 cephalic end the notochord has more the appearance of a rod 

 and is almost pinched off from the entoderm. On account of 

 the plane of the sections it is not possible to determine with 

 certainty the cephalic limit of the notochord. 



