THE EARLY HISTORY OF THE OVUM. 213 



In birds the first phenomenon of gastrulation is the forma- 

 tion of a crescentic or sickle-shaped groove at the margin of the 

 blastoderm, the anterior margin of which is directly comparable 

 to the rim of inflection in the elasmobranch. The edges of the 

 right and left halves of this groove coalesce as in the sharks, 

 and then the blastoderm grows backward beyond the primitive 

 streak thus formed, so that the streak comes to lie like an island 

 in the centre of the blastoderm. In the reptiles much the same 

 conditions occur as in the birds, except that the blastopore is 

 placed within rather than at the edge of the blastoderm. 



A feature to be noticed in all the foregoing types is that in 

 each case the embryo arisesi rom right and left portions, which at 

 first may be widely separate, and which 

 meet and fuse in the middle line. This . -^ ' , 



phenomenon of concrescence consists Q;;4%fe 

 in the formation of the dorsal portions 

 of the embryo, and all of the struc- p "^w^^ 

 tures there developed nervous sys- g-iKij 

 tern, myotomes, sclerotomes, vascular C M -: ;>i^ 

 system, and ccelomic structures SF 



from the union of the blastoporal lips. 1 

 This process is illustrated in our fig- 

 ures (Fig. 218) where the successive FIG. 219. Early blasto- 

 portions of the germinal ring (i.e., derm of hen ' s e gg> after K61- 

 edges of the blastopore) are shown liker ' r ' crescent ; *' primitive 



' groove ; o, area opaca ; /, area 



uniting to form the axial portion of pe iiucida. 

 the embryo. 



In the placental mammals the eggs are very small and the 

 amount of deutoplasm is small, consequently the eggs are holo- 

 blastic. In their segmentation and in the method of formation of 

 the germ layers these eggs present many peculiarities, which are 

 usually explained upon the hypothesis that the mammals have 

 descended from animals with large-yolked eggs, and that the 

 features in which they differ from the other vertebrates as well 

 as from other animals with holoblastic eggs are to be attributed 



1 Several authors (Sedgwick, Kastschenko, Morgan, and others) have criticised this 

 theory in one way or another, but the actual facts of development seem to negative their 

 arguments. 



