276 



s. GOTO ; 



Perihranchial cavities. — According to MacBride ['96, ]>. 08 1 J the 

 peribranchial s[):ices arise as diverticula from the enterocuel, althougli 

 it must be remai-ked that he expresses some uncertainty connected 

 witli them. In my opinion these spaces are likewise of mesenchyma- 

 tous orio-in, althouo-h it must be confessed that mv observations on this 

 c[uestion are not as complete as I could desire. In Fig. 3o I have 

 reproduced a small portion of a section (transverse) of a lateral wall of 

 one of the arms. It represents the earliest condition of the peribran- 

 chial spaces that has come under my notice. Here we see that there 

 is a more or less distinct layer of cells next the peritoneal epithelium, 

 similar to tlie latter. It is present at this stage (R = 2 mm.) in all 

 parts of the body. We see further that the peribranchial cavities are 

 formed Ijy the splitting of this cell layer. In tlie stage under con- 

 sideration it is a ring-like space encircling the diverticulum of the 

 enteroccel, which is to beci^me the central cavity of the dermal bran- 

 chia ; and at the tip the space left by the ring is spanned by a thin 

 cellular membrane, the direct continuation of the walls of the peribran- 

 chial cavity. This membrane must eitlier represent the original con- 

 dition of thino's or have been secondarily formed bv the o-fowth of the 

 branchial diverticulum of the enteroccel, in consequence of which the 

 walls of the central portion of the peribranchial cavity which was un- 

 til then single and continuous, w^ere applied to each other and have be- 

 come fused. My observations are confessedly incomplete on this point. 



As to the origin of the cell layer by the splitting of which the 

 peribranchial cavities are formed, I have found no indication that it is 

 derived from the ])eritoneal epithelium. It appears to me that it has 

 been formed by a special arrangement of some cells of the connective 

 tissue, as suof^'ested on the lower side of Fig. 33. 



October, 1897. 



