402 OUTLINES OF CHORDATE DEVELOPMENT 



vascular. The notochord is distinctly differentiated posteriorly, 

 and the embryonic mesoderm is only incompletely separated 

 into somatic and splanchnic layers (Fig. 159, B), although in 

 the extra-embryonic region there is a very wide coelom. 



A sagittal section through this embryo shows that it is some- 

 what arched over the dorsal surface of the y oik-sac , and that an 

 endodermal outgrowth, the rudiment of the allantois, is extend- 

 ing into the mesoderm of the body stalk (Fig. 161, E). 



Shortly after this, in an embryo measuring 1.8 X 0.9 mm. 

 (Keibel and Elze's embryo Klb.) the neural folds become very 

 prominently elevated and the head and tail regions project 

 slightly above the surface of the yolk-sac, as shown in Fig. 

 162, A. This figure shows also the persistent neurenteric 

 canal, and the very short primitive streak. Five or six pairs 

 of mesodermal somites are now present. 



The head region now commences to enlarge rapidly although 

 the neural groove is still open. In an embryo of 2.36 mm. 

 length (Kollmann's embryo, Bulk) illustrated in Fig. 162, B, 

 the body is concavely arched toward the yolk-sac, while the 

 head and tail regions show distinct downward flexures. The 

 elementary divisions of the brain are already indicated, and the 

 fore-brain is protecting downward from the anterior end of the 

 neural axis. Though not shown in the figure the paired rudi- 

 ment of the heart is present. About fifteen pairs of somites 

 are visible externally. 



As the embryo now begins to elongate rapidly it becomes 

 clearly folded off from the extra-embryonic structures, and the 

 opening of the yolk-sac out of the endodermal gut cavity of the 

 embryo becomes relatively, though not actually, narrower. 

 The yolk-sac thus appears to be attached to the embryo proper 

 by a narrow stalk, the yolk stalk, the connection of which, with 

 the embryo, is the yolk stalk umbilicus. 



By the time the embryo reaches a length of. 2.5 mm. (Koll- 

 mann's embryo, 2.5 mm., age given as thirteen to fourteen days, 

 but probably much older) the high neural folds have begun to 

 close together posteriorly (Fig. 163). The head region is con- 

 siderably enlarged and extends downward in front of the heart, 



