FOEMATTON AND FATE OF THE PEIMITIVE STEEAK. 477 



a purely passive agent in the formation of the primitive streak 

 in lizards ; and in the tortoise, according to Mitsuruki and 

 Ishikawa, the streak consists, in the first instance, mainly of a 

 mass of hypoblast or yolk, which they compare to the yolk- 

 plug of Amphibians. To this, however, it cannot correspond, 

 for we have already shown that the yolk-plug of Rana is a por- 

 tion of the ventral wall of the archenteron, whilst the primi- 

 tive streak is formed by the fusion of the lateral lips of a 

 deficiency in the posterior wall of the same cavity. 



But, whatever the mode of its formation may be, the streak 

 eventually becomes perforated, both anteriorly and posteriorly : 

 anteriorly by the neurenteric canal — Balfour, Hoff'mann, 

 Weldon, and Strahl;^ and posteriorly by the anus — Weldon. 

 It is thus, in all essential respects, comparable with the primi- 

 tive streak of birds. 



In mammals also a primitive streak is found. It is described 

 as commencing in the sheep (5) and in the shrew (25) as a 

 knob-like swelling of the epiblast in the posterior part of the 

 embryonic area. The epiblastic thickening extends backwards 

 and terminates in a tail-swelling. According to Bonnet (5), 

 Hubrect (25), and Hensen (20), the two primary layers fuse in 

 the streak, though it does not seem certain that the hypoblast 

 takes any part in the thickening. KoUiker (31) denies that 

 the hypoblast takes part in the formation of the streak in the 

 rabbit. Rabl (41) agrees with Kolliker, and Fleischmann (11) 

 makes a similar statement concerning the cat. Iii the primi- 

 tive streak of the guinea-pig (29) there is fusion of the layers 

 anteriorly, but posteriorly the thickened epiblastic ridge is 

 separate from the hypoblast. In the mole (17) all the layers 

 take part in the formation of the streak, but after a time they are 

 fused only at its anterior and posterior ends, whilst in the middle 

 the hypoblast forms a distinct layer. At the anterior end of 

 the mammalian primitive streak more or less distinct traces of 

 a neurenteric canal have been found in the rabbit by Strahl 



1 Strahl's statement (54), that in L. agilis the perforation occurs in the 

 middle of the streak, simply means, apparently, that there is a region of fusion 

 between the epiblast and hypoblast in front of the opening. 



