TRANSACTIONS OF THE SECTIONS. Dl 



denies this*) ; in fact, for a single peculiarity among them any which characterize 

 this primitive groove. 



Mr. Schafer, of University College, conceives this groove to be an infolding of 

 the epiblast or outer layer of the embryo, which then expanding laterally, forms 

 the mesoblast by a process of lateral cell-growth ; and his sections of the embryonic 

 Guinea-pig show clearly that from the fusion of layers which constitute the axis- 

 cord of His at the bottom of the primitive groove, the laterally outgrowing mass of 

 mesoblast can be seen to spring. Those who adopt the view of Kblliker and 

 Hensen, that the mesoblast is derived from the epiblast, will probably regard this 

 hypothesis, or some modification of it, with favour ; but if (as I am from my own 

 observations personally inclined) we regard the mesoblast as a derivative from the 

 inner embryonic layer, then we are forced to look elsewhere for an explanation of 

 the appearance. 



Our own English embryologist, Mr. Balfour, whom as a fellow member of the 

 British Association we delight to honour, has made this groove the subject of ex- 

 amination, and has correlated it with the change in position of the embryo on the 

 surface of the germinal disk. He has shown that in the egg of the bird or mammal 

 the embryo from the first appears in the centre of the blastoderm, while in more 

 generalized forms, such as the Shark, the first trace of the embryo is marginal ; 

 and from this position it gradually recedes, as in the growth of the blastoderm, 

 over the surface of the yolk the former extends more rapidly at each side of the 

 embryo than it does opposite the embiyo itself, and in the course of development 

 the two headlands which border the bay at whose fundus the embryonic shark 

 appears, approximate and unite mesially, forming a sutural fine leading from the 

 margin of the embryo to the edge of the blastoderm. He considers that in the 

 highly specialized form of the bird or mammal the primitive groove is the heirloom 

 of this ancestral change in condition; and if we correlate with this historic evi- 

 dence such appearances as the marginal notch in the germinal disk, described by 

 Professor llaubert of Leipzig, and considered by him as the ideal hinder edge of 

 the primitive groove, we can, I believe, make out an almost overwhelming c, 

 favour of this theory, more especially as we cannot but believe that the change 

 which Balfour postulates has taken place, and that this, in accordance with mor- 

 phological analogy, should leave some trace of its having existed behind it. 



Prof. Baubert, who has also written several papers on this subject, has con- 

 sidered this groove to be a trace of the opening of invagination of the hypoblast, a 

 groove continuous with the extremity of the blastopore, or primitive mouth, of 

 which, indeed, he supposes it to form a part. This view is based principally on 

 the dorsal extension of the blastopore in Amphioxus and in Ascidians§, and on the 

 general evidence of the origin of the hypoblast by invagination, as well as on the 

 observed fusion of the hypo- and mesoblast at the hinder end of _ the groove and 

 their separateness anteriorly. As the blastopore is placed at the hinder end of the 

 embryo, at the bottom of the bay of blastoderm, and as the neural and hypoblastic 

 canals are here confluent, as are the primitive and medullary grooves, this may be 

 one factor in the formation of the groove, at least of its anterior extremity. Into 

 the larger question of the origin of the mesoblast time forbids our entering. 



I shall now very briefly direct your attention to a second series of embryological 

 res. -arches, viz. those on the earliest formation and ontogeny of limbs. 



In his remarkable and suggestive series of papers on the development of Elasmo- 

 branchs, Mr. Balfour || describes the first appearance of the limb to be a thickened 

 rid"-e of epiblast, more prominent in front and behind than mesially, and forming 

 a continuous lateral fin. In later stages of development themesoblast beneath this 

 ridge thickens, projects, and, being substituted for the epiblast, forms the chief 

 substance of the limb as it afterwards exists. In the rabbit-embryo Kollikerf like- 



* Entwickelungsgeschichte, p. 125. 



t Morpholog. Jahrbuch, Ed. ii. p. 571, Taf. 37. fig. 11. 



j Qegenbaur'a Morpholog. Jahrb. 13d. ii. p. 5f>0. 



§ KowalewBky, " Embryologischo Studien, Mem. Acad. St. Pctersb. 1871, p. 30. 



i| Journal of Anatomy and Physiology, vol. xi. p. 132. 



"|[ Entwickelungsgeschichte, 1876, p. 211. 



