ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 315 



gives rise in part to tlie mesoblast of the allantois, and ventral part 

 of the tail of the embryo, and in part to the vascular structures 

 found in the area pellucida. 



With reference to the formation of the mesoblast of the primitive 

 streak, the authors' conclusions are practically in harmony with 

 those of Koller ; except that Keller is inclined to minimize the share 

 taken by the hypoblast in the formation of the mesoblast of the 

 primitive streak. 



Gerlach, with reference to the formation of this part of the meso- 

 blast, adopts the now generally accepted view of Kolliker, according 

 to which the whole of the mesoblast of the primitive streak is 

 derived from the epiblast. 



As to the derivation of the lateral plates of mesoblast of the 

 trunk from the hypoblast of the anterior part of the primitive streak, 

 the authors' general result is in complete harmony with Gerlach's 

 results, although in their accounts of the details of the process they 

 differ in some not unimportant particulars. 



2. As to the origin of the notochord, their main result is that 

 this structure is formed as an actual thickening of the primitive 

 hypoblast of the anterior part of the area pellucida. It unites 

 posteriorly with a forward growth of the axial tissue of the primitive 

 streak, while it is laterally continuous at first, both with the meso- 

 blast of the lateral plates and with the hypoblast. At a later period 

 its connection with the mesoblast is severed, while the hypoblast 

 becomes differentiated as a continuous layer below it. 



As to the hypoblastic origin of the notochord, they are again in 

 complete accord with Gerlach, but differ from him in admitting that 

 the notochord is continuous posteriorly with the axial tissue of the 

 primitive streak, and also at first continuous with the lateral plates 

 of mesoblast. 



The authors add : — " The account we have given of the forma- 

 tion of the mesoblast may appear to the reader somewhat fantastic, 

 and on that account not very credible. We believe, however, that if 

 the view which has been elsewhere urged by one of us, that the 

 primitive streak is the homologue of the blastopore of the lower ver- 

 tebrates, is accepted, the features we have described receive an 

 adequate explanation. 



" The growth outwards of part of the mesoblast from the axial 

 line of the primitive streak is a repetition of the well-known growth 

 from the lips of the blastopore. It might have been anticipated that 

 all the layers would fuse along the line of the primitive streak, and 

 that the hypoblast as well as part of the mesoblast would grow out 

 from it. There is, however, clearly a precocious formation of the 

 hypoblast ; but the formation of the mesoblast of the primitive streak, 

 partly from the epiblast and partly from the hypoblast, is satisfactorily 

 explained by regarding the whole structure as the blastopore. The 

 two parts of the mesoblast subsequently become indistinguishable, 

 and their difference in origin is, on the above view, to be regarded 

 as simply due to a difference of position, and not as having a deeper 

 significance. 



Y 2 



