52 WALTBE HEAPB. 



Beneden (No. 5) describes that portion of the inner mass which 

 remains after the hypoblast is separated from it, as mesoblast, 

 and states that it retreats to the hinder end of the embryonic 

 area, becomes secondarily united with the epiblast, and gives 

 rise to the mesoblast of the embryo. 



Rauber (No. 21), Kolliker (No. 16), Hensen (No. 12), and 

 Lieberkiihn (No. 19), argue that the mesoblast arises first in 

 the primitive streak. Kolliker considers that the epiblast 

 alone gives rise to it, and that after being formed in the primi- 

 tive streak it spreads, eventually, over the whole embryonic 

 area, and also supplies the mesoblast of the area opaca. From this 

 author's statement I gather he considers the primitive streak 

 arises first in the end knob (Endwuldst), and extends from 

 thence forwards. 



Lieberkiihn differs from Kolliker, and agrees with Hensen, 

 in that he derives the mesoblast of the primitive streak from 

 both epiblast and hypoblast; while Hensen differs from the 

 other observers mentioned, in considering a certain amount of 

 the mesoblast of the area opaca to be formed in situ from 

 hypoblast. 



Summary. — My own observations lead me to differ entirely 

 from Beneden as to the formation of mesoblast,and to agree with 

 the other observers mentioned above in concluding that it is first 

 formed in the primitive streak. I cannot, however, accept 

 KoUiker's statement, that the epiblast is alone responsible for its 

 production, and that it is first formed in the hind knob. I consider 

 that the hind knob is formed some time after the first portion 

 of the primitive streak, and that the formation of the latter 

 takes place from before backwards instead of from behind 

 forwards, as this observer states ; my main reason for this 

 being, the universal presence at the anterior end of the 

 primitive streak of an indication of the involution of the 

 epiblast. 



Again, I agree most distinctly with Hensen and Lieberkiihn 

 in regarding the epiblast and hypoblast as the originators of 

 mesoblast at the front end of the primitive streak, but I must 

 differ from them, and from Kolliker (loc. cit.) and Schafer 



