118 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



right. Separating these prominent folds (plate 3, figs. 2 and 3) lies, in their caudal 

 halves, the anterior end of what we may call the primitive groove, here very deep 

 and narrow. The groove between these folds in their cephalic portion is much 

 shallower and finally lost. Near the posterior end of this shallower groove, which 

 is continued forward without distinct interruption from the primitive groove, but 

 in a plane slightly to the right, lies the minute dorsal opening of the archenteric 

 canal so-called chordal canal slightly to the left in the bottom of the groove. 



The anterior third of the blastoderm is in general slightly convex, but its sur- 

 face is broken up by many small, irregular folds to which one can attach no signifi- 

 cance. It is certain that the ectoderm in the anterior half of the embryo has suffered 

 more distortion than any other part. The result has been an obliteration, as far as 

 they may have been indicated, of the early medullary folds anteriorly, coupled with 

 what seems to be their accentuation and prolongation posteriorly. That these last- 

 mentioned folds, occupying the center of the blastoderm, have anything to do with 

 the medullary folds is, considering the stage of development, very doubtful. The 

 posterior ends of these folds, especially on the right, have a remote resemblance 

 to the caudal lobes of a later date. 



Fio. 2. Blastoderm and axial struc- 

 tures in dorsal view. X about 

 40. For more exact details of the 

 head process see text-figures 4 and 

 5. The location of the cloaca! 

 membrane is indicated by dotted 

 outline. Close to the posterior 

 tip of the amnion is the allan- 

 tois; in section and near it, in the 

 body-stalk, are sections of two 

 vessels. The marginal lines have 

 reference to the location from 

 which were taken the photo- 

 graphs (P.) and drawings (D.) 

 shown on plates 2 and 3. 



p 375 - 



P 380 



P 401 - 

 f 406 - 



0.394+ D.40I D406* 



The amnion lies close to the embryonic ectoderm anteriorly, while farther back 

 it is lifted high above it by being incorporated in the body-stalk. Any indications 

 of the presence or recent disappearance of an amniotic duct as noted by Grosser 

 (1913) and Strahl and Beneke (1910) are wanting. This amniotic duct may very 

 well be one of those instances of embryonic variation referred to above, not only 

 variable but finite possibly very transient, and in this same category may be placed 

 a peculiar feature of our embryo to which we would here draw attention. As can 

 be seen in the accompanying illustrations, the caudal tip of the amnion lies in very 

 close proximity to the allantois, a short distance above the connection of the latter 

 with the yolk-sac. In at least two sections there is a very distinct though liny, 

 narrow out-pocketing of the amniotic cavity toward the allantois (not shown in the 

 figures). Here the epithelium of the amnion is of a low cuboidal type in contrast 

 with its squamous character in the immediate vicinity. There is no connection 



