FORMATION AND FATE OF THE PRIMITIVE STREAK. 105 



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 portion of 

 the ventral wall of the archenteron, whilst the primitive 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, Hoffmann, Weldon, and 

 Strahl; 1 and posteriorly by the anus — Weldon. It is thus, in all 

 essential respects, comparable with the primitive 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. Kolliker 

 (31), denies that the hypoblast takes part in the formation of the 

 streak in the rabbit. Babl (41) agrees with Kolliker, and Fleischmann 

 (11) makes a similar statement concerning the cat. In the primitive 

 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 (55), in the mole by Lieberkuhn (35) and Heape (17), 

 in the sheep by Bonnet (5 and 6), and in the bat by Van Beneden (3). 

 In the anterior wall of the opening the chorda and neural epiblast 

 are fused, and laterally and posteriorly the mesoblast hangs in 

 connection with the margins of the streak. 



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. 



