SALPA — DEVELOPMENT OF THE BUDS ON THE STOLON. 



499 



ectoderm of the stolon must, however, be considered probable for the 

 neural tube. We should add that the examination of these onto- 

 genetic processes in the stolon is exceedingly difficult.* 



B. Development of the Buds on the Stolon. 



It has already been shown (p. 495) that the buds arise through 

 transverse constriction of the stolon (Fig. 266). The stolon is in this 

 way cut up by a kind of transverse fission into a series of consecutive 

 individuals which continuously inci'ease in size while remaining con- 

 nected together by the narrowed parts of the stolon. The buds thus 

 arise in Salpa in exactly the same way as in Pyrosoma (Figs. 253 

 and 256 C). Only in later stages do the buds of the Salpidae undergo 

 important changes of position which lead to their biserial arrangement 

 on the stolon. In order 

 clearly to understand 

 these changes, we must 

 first recall the condition 

 of the Pyrosoma stolon. 

 According to the more 

 detailed account given 

 <>n p. 484, the individuals 

 are arranged on the 

 Pyrosoma stolon in a 

 single row, one behind 

 the other. The orienta- 

 tion of each individual 

 resembles that of the 

 parent. The haemal or 

 ventral side of the buds 

 (marked by the position 



of the endostyle, es) is directed towards the distal end of the stolon. 

 The right half of the body of every individual agrees in position with 

 the right side of the parent; the left half of the bud corresponds to 

 the left half of the parent and has been produced by the left half of 

 the stolon, just as the right half of the bud has been produced by 

 the right half of the stolon. 



i 



Fig. 267. — Diagrammatic longitudinal section 

 through the stolon of a Pyrosoma (constructed by 

 Brooks after Huxley and Kowalevsky). P, 

 parent-individual ; /, //, ///, first second and 

 third buds ; b, branchial-sac ; c, atrium ; d, 

 alimentary canal ; ec, ectoderm of the connecting 

 strand ; en, entoderm of the same ; es, endostyle ; 

 /(. nervous system; o, segment of the genital 

 strand; s, young stolon of the third individual. | 



* [According to Bbooks (No. I.), the nerve-tube and the two peribranchial 

 tubes are probably derived from the ectoderm of the stolon ; the upper blood- 

 sinus is continuous with the cavity of the heart, and the lower sinus with the 

 body-cavity of the nurse-form. — Ed.] 



