434 WALTER UEAPE. 



these become hypoblast cells. In the axial line no such 

 change occurs, and the mass of cells existing there, is con- 

 tinuous behind (by means of the axial rod described on p. 430) 

 with the front end of the primitive streak, and continuous 

 laterally with both the hypoblast and the mesoblast. Fig. 42, 

 although it is a section througli a considerably older embryo, 

 represents these relations fairly accurately. 



The axial mass of cells eventually gives rise to the noto- 

 chord. The lateral mesoblast may be called hypoblastic meso- 

 blast in accordance with its origin, and to distinguish it from 

 the mesoblast of the primitive streak. The lateral masses of 

 liypoblastic mesoblast adjoin posteriorly the mesoblast of the 

 primitive streak, and it does not appear to me to be possible, 

 with the existing methods of discrimination, to determine the 

 exact extent of either layer ; roughly, however, we may say 

 that the front end of the primitive streak is the boundary line. 

 At the stage of development now reached the embryo maybe 

 compared with that of Amphioxus, as far as its structure is 

 concerned in front of (he primitive streak ; two masses of 

 mesoblast are formed from the hypoblast laterally and the 

 axial hypoblast thickens and gives rise to the notochord. The 

 latter is similar to the median diverticulum of the enteric 

 cavity of Amphioxus, and the lateral masses of mesoblast to the 

 mesoblast of the united diverticula on each side in that animal ; 

 the lateral diverticula do not, however, appear, but the median 

 one is, as we shall see, formed later. 



With regard to the embryonic vesicle it is much larger than 

 in the previous stage, and no longer projects into the mouths 

 of the uterine glands, but is exceedingly closely applied to the 

 uterine epithelium, so closely that some of the latter is gene- 

 rally pulled away from the uterus when the ovum is obtained 

 whole. Fig. 53 is a section of a portion of the vesicle wall 

 which is formed of flattened epiblast only, and of the uterine 

 epithelium to which it is closely adherent. 



Historical. — Various accounts have been given by different 

 observers as lo the origin of mesoblast in mammalian embryos. 



