344 JORDAN. [Vol. VIII. 



is shown in Figs. 49-5 1 . Here the primitive groove is deep, 

 and is interrupted in the middle by the still exposed yolk-plug. 

 Fig. 58 is a horizontal section through a stage somewhat older 

 than that shown in Fig. 42, and represents the primitive streak 

 and primitive groove. This condition results, it is evident, 

 from the fusion of the lateral lips depicted in Fig. 63. A still 

 later aspect of the primitive streak is represented in Fig. 65. 

 This is a section through the hind end of an Qgg slightly 

 younger than that shown in Fig. 44, and shows the some- 

 what dorsal position of the streak at this time. The neuropore 

 has by this time vanished, and the primitive streak in conse- 

 quence extends with unbroken continuity in front of the anus. 

 The section figured is the twelfth in front of the anus, and 

 there are ten sections more before the anterior limit of the 

 primitive streak is reached. Fig. 59 is the eighteenth section 

 anterior to Fig. 65, and Fig. 53 is the thirty-seventh anterior 

 to Fig. 59. 



The primitive streak and groove as they appear in surface 

 views are shown in Fig. 42 as the dark line between the 

 neuropore and anus. Later these surface features cannot be 

 detected ; the groove is completely obliterated, but sections 

 reveal to us the presence of a primitive streak running both 

 backwards and forwards from the anus (Fig. 65). 



It is this primitive streak in front of the aniLS which was 

 looked upon by Miss Johnson ('84), so far as I can gather from 

 her figures, as lying in front of the blastopore. The two state- 

 ments carry quite a different signification, as will be evident 

 to those who have followed my description. I am unable, 

 however, to account for the great forward extension of the 

 primitive streak represented in some of Miss Johnson's figures. 

 In Diemyctyhis the fusion of layers by no means reaches so far 

 forward as "the middle of the embryo." No more can I 

 reconcile my observations with those of O. Schultze ('88) on 

 Rana fusca. Schultze depicts a primitive streak which " von 

 der dorsalen Urmundlippe sich nach dem Kopfe hin allmahlich 

 ausdehnt, und wachst also der Primitivstreif auch bei den 

 Amphibien von hint en nach vorn." (p. 330.) However it 

 may be in these animals, in Dicmyctylus, I am confident that 



