WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 67 



body of the embryo will render this flexure even more typical and conspicuous. 

 The appearance of the fore-brain is here quite different from the condition as 

 usually figured, where the neural folds are represented as being strongly convex. 

 The remainder of the nervous system, represented by the cord, is a deep, open 

 furrow in the back of the embryo. Caudally, there is a rather sudden decrease in 

 the thickness of the ectoderm forming the folds ; the groove also becomes very faint 

 and shallow, but can be traced as far back as the primitive node. About the middle 

 of the dorsal concavity, in the region which may be taken as upper cervical or lower 

 medulla, the interval between the crests of the neural folds is less than elsewhere, 

 and here the closure will first begin. 



In the surface ectoderm on either side, about opposite the middle of the rhomb- 

 encephalon, are two not very sharply delimited, moderate thickenings, which are 

 the otic plates. They are in the main quite symmetrical, somewhat elongated, and 

 measure about 0.08 mm. in diameter. The basal surface of the ectoderm here is 

 cleaner and sharper than in the adjacent body-wall and hence more like the neural 

 folds. These thickened areas are somewhat more extensive than shown in figures 

 1 and 2, but all of the material may not be found later in the otic vesicles, some being 

 utilized in the separation off and covering in of the same. As indicative of a gang- 

 lion crest we may note that toward the anterior limit of the rhombencephalon 

 there appear two fairly distinct and symmetrical collections of cells, of rather short 

 extent, capping the neural folds on either side — the primordia of the ganglia of the 

 fifth cranial nerves (figs. 1 and 2). 



At the very posterior end of the embryo, occupying the lower half of the caudal 

 slope of this part of the body, is the primitive streak. From the primitive node to 

 the cloacal membrane it measures 0.13 mm. in length and is marked by a broad, 

 shallow groove. At the primitive node there is a rather diffuse, loose connection 

 between the three germ layers; the ectoderm is here distinctly thickened and faint 

 indications of the remains of the dorsal opening of an archenteric canal can be made 

 out. The latter seems to penetrate about half way to the entoderm and may there- 

 fore be in relation with the posterior end of the chorda, but nothing definite as 

 regards this can be made out. Ventral and lateral to the primitive streak there is 

 an abundance of rather loose mesenchyme. 



The anterior (later ventral) surface of the head is regularly convex, with the 

 median groove and beginning stomodeum as already noted. On either side, where 

 this surface passes into the lateral surfaces of the head, are two quite distinct verti- 

 cal grooves, the one on the left being rather deeper, shorter, and nearer the median 

 line. Internally there is, on the left side, a shallow, poorly defined extension of 

 somewhat thickened entoderm toward the ectodermic groove. The lateral surface 

 of the head region is marked on the left side by two distinct, vertical, parallel 

 furrows (fig. 1). The anterior one is short and is simply a fold involving the entire 

 thickness of the body-wall, projecting internally into the pericardial ccelom. The 

 other groove is much longer and if continued upward would cut the neural tube in 

 the region of the mid-brain flexure. Corresponding with this posterior groove there 

 is a similar one, on the right side, best marked low down, where its ectoderm lies 

 close to the pharyngeal entoderm. 



