136 



A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



In a study of two embryos of 3 mm., Hammar 17 reports the tuberculum impar 

 present in only one. 



Between the ventral pharyngeal grooves of the second and third pouches is 

 a second medial elevation in the form of a transverse ridge. From its position 

 I believe it to be the so-called "copula" which, according to His and others, helps 

 to form the tongue. 



A third prominent elevation is found behind the ventral pharyngeal groove 

 of the third pouch. It marks the point where the pharynx turns sharply caudal- 

 ward. This probably is the area which Grosser had denoted the "cardiac swell- 

 ing" and which he states goes into the formation of the larynx. 



,ch.d. 



ao.d.s. 



ch.d. 



ao.d.s. 



P.P.5(?) 



meso. 

 Fin. 2. Portion of section 70. Xllifi diameters. 



I'll.. :!. Portion of sec-lion 84. X160 diameters. 



The lateral borders or wall of the pharynx extend outward into three distinct 

 pharyngeal pouches. Of these, only the first and second reach the ectoderm. The 

 first pair of pouches is situated only a short distance behind the oral membrane. 

 They are flattened from before backward, the left being somewhat broader than 

 the right. In studying the first pouch of young embryos, Grosser 15 describes an 

 invagination of its epithelium into the pharyngeal cavity as follows: 



"In the region of the first pouch there projects ventrally or caudally from the closing 

 membrane into the pharyngeal lumen an irregularly knobbed process filled with meso- 

 derni .... It disappears quite early and may perhaps be interpreted as a rudimentary 

 internal gill." 



I have looked carefully in the region designated by Grosser for the structure 

 which he describes, but have been unable to find any definite indication of it. 

 A slight irregularity of the epithelium, however (more distinct on the left side 

 than on the right), corresponds with it in position. 



The second pouch is somewhat larger than the first and is flattened dorso- 

 ventrally. A distinct ventral diverticulum can be seen. The third pouch is 

 more rounded in form and ends bluntly in the mesenchyma, falling somewhat 



