WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 69 



Externally in the ectoderm there is a well-marked linear depression of con- 

 siderably greater extent than the small furrow just noted, but having the same 

 direction. Here over an extent of about 0.04 mm. the ectoderm and entoderm are 

 quite close together, no mesenchymal cells intervening, but the two layers are not 

 in actual contact. Just in front of this groove lies one of the anterior, finger-like 

 prolongations of the pericardial ccelom, while still farther forward are found the 

 beginnings of the ventral aorta and first arch. However, on this (the right) side 

 there are other indistinct furrows in both the ectoderm and entoderm, but they 

 sustain no relation to each other and we would look upon the condition described 

 above as an early indication of the first cleft. On the left side this small ventral 

 furrow is not as definite, the gut here being more evenly expanded. Externally, 

 as previously noted, there is a groove in the ectoderm very similar to that on the 

 right, but everywhere between it and the entoderm there is mesenchyme present. 

 At no point is there any well-defined variation in thickness in either ectoderm or 

 entoderm corresponding with these grooves. The right side is slightly in advance 

 of the left and the same appears to be the case in the two other embryos we have 

 mentioned, No. 391 and No. 1201. 



The buccopharyngeal membrane is well defined, measuring about 0.05 mm. 

 from side to side and slightly more from above downwards. The ectoderm and 

 entoderm over this area are in intimate contact; the former shows here a definite 

 increase in thickness over the neighboring ectoderm, while the entoderm is not 

 locally changed, but is distinctly thicker over the floor and side-walls of the pharynx 

 than elsewhere. Externally the membrane lies immediately above the reflections 

 of the somatopleure from the body to form the anterior limit of the pericardial 

 cavity. It occupies the floor and side-walls of a shallow depression, which is con- 

 tinued by a definite furrow over the anterior end of the head into the open neural 

 tube. The membrane seems to be rather widely separated from the first cleft, i. e., 

 the latter appears quite far back on the side of the head, but later development will 

 approximate the two considerably. 



In the mid- ventral fine, just anterior to the point where the deep ventral 

 trough of the fore-gut begins to widen out, is the primordium of the thyroid. It is 

 a short, well-circumscribed outgrowth from the floor of the pharynx, indicated 

 internally by a very short but well-defined lumen. In the model this outgrowth is 

 located slightly to the right and directed also quite definitely in the same direction. 

 The relation to the first internal pocket has been already noted. The thyroid is 

 here in close relation with that part of the cardiac plexus which will later give rise 

 to the ventral aorta and beginning of the first arch, in practically the same location 

 in which we have described it for a much older stage (1907, p. 548). In No. 391 

 there seems to be an early thyroid in the same position as regards the heart, but 

 this can not be determined with certainty on account of defects in the sections. 



It is unfortunate that no definite statements can be made concerning the 

 chorda, particularly as regards its anterior and posterior extremities. In addition 

 to the unfavorable plane of the sections through the posterior two-thirds of the 

 specimen there is (particularly in front of this) considerable loss of tissue involving 



