354 



GENERAL EMBRYOLOGY 



of the median thickening, known as the primitive streak or 

 embryonic rudiment, by a process of gradual fusion posteriorly 

 (Figs. 162, 163). But in this process of coming together, which 

 may better be termed confluence (Sumner) than concrescence, 

 the materials from the two sides of the germ ring are fused into 

 a mass which is largely ^differentiated, and out of this the 

 rudiments of the embryo appear, by a process of differentiation 

 which occurs largely after confluence. One side of the germ 

 ring contains not a half of the embryo, but the substance out of 

 which, later, a half of an embryo forms. This process of 

 differentiation is progressive and commences of course in that 



en. 



FIG. 164. Sagittal section through the hinder end of a fish embryo (Serranus), 

 showing the undifferentiated primitive streak, anterior to which the structures 

 of the embryo are being differentiated. From H. V. Wilson, a.p. (v.l.), anterior 

 margin of blastoderm or ventral lip of blastopore, after having grown entirely 

 around the yolk mass, bl., blastopore; ec, ectoderm; en., endoderm; g.r., germ 

 ring; k.v., Kupffer's vesicle; nc., notochord; nr. ch., nerve cord; p., periblast; 

 pp. (d.l.), posterior margin of blastoderm (dorsal lip of blastopore); pr. str., 

 primitive streak. 



part of the primitive streak formed first, i.e., its morphological 

 anterior end (Fig. 163). Then as the primitive streak lengthens 

 posteriorly, the extent of the differentiated region at its anterior 

 end similarly increases posteriorly, roughly keeping pace with 

 the process of elongation (Fig. 164). The primitive streak 

 thus may be regarded as a region which moves backward, 

 receiving posteriorly the diverging limbs of the germ ring, and 

 leaving anteriorly the differentiated rudiments of the embryo. 

 Soon after the germ ring is completely closed or contracted, 

 the primitive streak becomes wholly differentiated, and the 



