THE EARLY DEVELOPMENT OF THE FROG 119 



the iwuml crests. _ These are concerned later in the development 

 of the nerves, which, together with some additional details of 

 this period, will be described in the next chapter. 



At the posterior end of the embryo, the relations of the blasto- 

 pore (germ ring) and neural folds are of considerable importance 

 from a comparative point of view. We left the blastopore in 

 the form of an elongated slit on the postero-dorsal surface of the 

 embryo. The lateral walls then approach, about the middle of 

 the slit, and finally it is there constricted completely, so that the 

 blastopore is divided into two small openings, an upper and a 

 lower (Fig. 38, A, B). The medullary groove extends forward 

 from the upper opening, which remains open, leading directly 

 into the archenteron (Fig. 37). The lower_opening is soon 

 closed by the fusion of its lips. The fusion involves only the 

 ectoderm and endoderm of the region, and occurs some distance 

 below the surface, so that a pit-like depression is left on the 

 surface, lined with ectoderm ; this is the proctodcK urn . When the 

 posterior ends of the neural folds form, they extend into, or 

 rather out from, the middle regions which have fused, and as 

 they become elevated and form the neural tube they cover over 

 the upper blastoporal opening (Fig. 38, C) which thus becomes 

 the 'tie ure> t (eric canal, like that of Amphioxus or other Chordates, 

 and similarly formed, putting the neurocoel and gut into com- 

 munication. The lateral margins of the blastopore are formed 

 of the remains of the germ ring, and when they meet, dividing 

 the blastopore, they form a median cell mass in which ectoderm, 

 endoderm, and mesoderm are fused in a more or less undifferen- 

 tiated mass. This mass can no longer be called the germ ring, 

 although it is really equivalent to the lateral parts of this; it is 

 known as the primitive streak, and the groove that remains for 

 a time on its surface, indicating its origin from originally sepa- 

 rate lateral portions, is the primitive groove. The primitive 

 streak and groove of the frog are homologous with the similarly 

 named structures in Amphioxus and in the Amniota. The cells 

 of the primitive streak continue to multiply rapidly, and bud 

 forth strands of ectoderm into the neural folds and upon the 

 surface of the body, mesoderm into the lateral bands, and en- 



