THE LAYERS OF THE BLASTODERM. 35 



primitive origin of the mesoblast, either from the epiblast or 

 hypoblast, or from both. In the chick the mesoblast cells 

 are formed directly from the ultimate products of segmentation. 

 From having a secondary origin in most invertebrates the 

 mesoblast comes to have, in the chick, a primary origin from the 

 segmentation spheres, precisely as we find to be the case with 

 the nervous layer in osseous fishes. It is true we cannot tell 

 which segmentation-cells will form the mesoblast, and which the 

 hypoblast ; but the mesoblast and hypoblast are formed at the 

 same time, and both of them directly from segmentation spheres. 



The process of formation of the mesoblast in Loligo, as 

 observed by Mr Ray Lankester (Annals and Magazine of Natural 

 History, February, 1873), is still more modified. Here the 

 mesoblast arises independently of the blastoderm, and by a 

 process of free cell-formation in the yolk round the edge of the 

 blastoderm. If Oellacher's observations in reference to the 

 origin of formative cells are correct, then the modes of origin 

 of the mesoblast in Loligo and the chick would have nothing in 

 common ; but if the formative cells are in reality derived from 

 the white yolk, and also are alone concerned in the formation of 

 the mesoblast, then the modes of formation of the mesoblast in 

 the chick would be substantially the same as that observed 

 by Mr Ray Lankester in Loligo. 



No very important changes take place in the actual forms 

 of the cells during the next few hours. A kind of fusion takes 

 place between the epiblast and the mesoblast along the line 

 of the primitive streak forming the axis-string of His ; but the 

 line of junction between the layers is almost always more or less 

 visible in sections. In any case it does not appear that there is 

 any derivation of mesoblast cells from the epiblast ; and since 

 the fusion only takes place in the region of the primitive groove, 

 and not in front, where the medullary groove arises (see succeed- 

 ing paper), it cannot be considered of any importance in reference 

 to the possible origin of the Wolffian duct, &c., from the epiblast 

 (as mooted by Waldeyer, Eicrstock nnd Ei, Leipzig, 1870). 

 The primitive groove, as can be seen in sections, begins to 

 appear very early, generally before the twelfth hour. The 

 epiblast spreads rapidly over the white yolk, and the area 

 pellucida also increases in size. 



32 



