78 THE DEVELOPMENT OF THE CHICK 



The mode of origin of the mesoderm of birds has been a very puzzling 

 question as is proved by the numerous views that have been in vogue 

 from time to time. One of the earliest views was that the mesoderm 

 arose by splitting of the primary entoderm (Remak). This view sur- 

 vives in part even at the present time (mesoblast of the opaque area). 

 Balfour believed that the mesoblast in the region of the embryo " ori- 

 ginates as two lateral plates split off from the primitive hypoblast," and 

 that the primitive streak mesoblast is extra-embryonic, or at most enters 

 into the formation of mesoblast of the extreme hind end of the embryo 

 (allantois mesoblast in part). This view is found in the "Elements of 

 Embryology" of Foster and Balfour. A third view, now of historical 

 interest only, was that the mesoblast cells arose peripherally and mi- 

 grated between the two primary germ-layers (Peremeschko, Goette). 

 The latter author even attempted to derive the primitive streak from 

 an aggregation of such inwandering cells. The view that the primitive 

 streak arises as a thickening of the ectoderm and that it is the source 

 of all the mesoderm was first stated by Kolliker, and has been accepted 

 by Hertwig, Rabl, and many others. It may, indeed, be regarded as 

 definitely established for the embryonic mesoblast. Others, however, 

 believe with His that the mesoblast of the opaque area arises by delam- 

 ination from the germ-wall; this question is discussed beyond. It should 

 also be noted that it is probable that the primitive embryonic mesoblast 

 is supplemented in certain regions at later stages by cells proliferated 

 from both entoderm and ectoderm, particularly in the region of the 

 head. (See pp. 116, 117.) 



In early stages of the primitive streak the mesoblast cells 

 are relatively sparse and bear every appearance of migrating 

 separately. But as the ingrowth progresses and the cells become 

 more numerous, the mesoderm becomes converted into coherent 

 plates. These are wedge-shaped, the central broad ends fused 

 with the primitive streak and the narrow margins extending 

 laterally (Figs. 40 A, B, C). They soon overlap the margin of 

 the opaque area and thus is produced a three-layered portion of 



FIG. 40. Three transverse sections of a late stage (corresponding to about 

 Fig. 44 B), through the head-process and primitive streak of a chick embryo. 



A. Near the hind end of the head-process. 



B. Through the primitive pit. 



C. A short distance behind the center of the primitive streak. The region 

 between the lines A-A and B-B is represented under a high magnification 

 in Fig. 41. 



Bl. I., Blood island, coel. Mes., Coelomic mesoblast. Ect., Ectoderm. 

 Ent., Entoderm. G. W., Germ-wall, med. pi., Medullary plate. Mes., Meso- 

 derm. N'ch., Notochord. pr. f., Primitive fold. pr. gr., Primitive groove, 

 pr. p.. Primitive pit. 



