Primitive Streak and Dorsal Notochoj'dal Opening; 21 



entoderm, which has an axial thickening, is fused with the ento- 

 derm of /3 at the point marked by an asterisk. The notochor- 

 dal canal of /3 has opened below (Plate V. Fig. 26) between 

 the points endrm. v. /8 and eu'dnn. d. /3. In Figure 27 (Plate 

 VI.) the notochordal canal of yS is still losing its anterior floor 

 at the points endrm. v. /3 and en'drm. d. /3, but is cut off from the 

 yolk by the presence of the lower-lying entoderm of a {en'drm. a). 



From the preceding description and the figures cited it is 

 evident that it will be most difficult to determine the anterior 

 limit of the primitive groove of individual a. If we adopt the 

 interpretation that the primitive groove is limited to those sections 

 in which ectoderm and entoderm are fused, then it is continuous 

 with the medullary groove. We find, moreover, a groove on the 

 streak in front of the small invagination, as well as on that behind 

 it ; in other words, the invagination falls within the streak, as 

 Strahl has observed for Lacerta agilis ('82, pp. 242 and 246) and 

 Lacerta viridis ('83, p. 9). 



Miss Johnson ('84, p. 663) states that in the newt — Triton 

 cristatus — the primitive groove extends throughout the whole 

 length of the medullary groove, and that in one instance it even 

 extended anterior to the medullary groove and there ended in a 

 triangular pit. Keibel ('93, p. 117) has recently maintained that 

 the primitive streak of the embryo pig at one stage reaches the 

 anterior end of the chorda, — hence the anterior end of the future 

 embryo, — and then recedes. Still more recently he ('94, p. 158) 

 expresses the opinion that the primitive groove of the embryo pig 

 in one stage extends into the region in which the medullary groove 

 will later develop. According to the experiments of Hertwig ('92) 

 on the frog, the blastopore extended primarily along the whole 

 axis of the embryo. Roux ('88, p. 143) states that in the frog 

 the chorda is formed by the fusion of the free margins of the 

 blastopore. Kcilliker ('82) for the rabbit, Heape ('83) for the 

 mole. Bonnet ('84) for the sheep, Hubrecht ('90) for the mole, 

 and van Beneden et Julin ('85) for the cat, have figured a forward 

 extension of the primitive streak, but not of such a length as 

 Keibel ('93, p. 120) has stated it to be in the case of the pig. 

 According to Gasser the streak in the case of the chick takes part 

 in the formation of the embryo. He ('79, p. 26) also observes a 

 shortening of the streak, which, in agreement with the observations 

 on Mammalia, takes place at its anterior end. In his stages two 



