1923] 



Bud Development of a Simple Ascidian 



93 



Three organ systems which very early make their appearance are 

 the pericardium and heart, the hypophysis and its associated struc- 

 tures, and the gonads. These organs all develop as Lefevre- has de- 

 scribed, from proliferating areas of the wall of the inner vesicle to 

 which possibly free mesenchyme cells add themselves. My observa- 

 tions confirm Lefevre 's account. The primordium of the hypophysis 

 appears at the extreme anterior end of the inner vesicle. The mass 

 of cells which is produced by proliferation later acquires a lumen and 

 assumes the form of a tube which lies along the mid-dorsal line. The 

 ganglion develops by proliferation from the dorsal wall of the hypo- 

 physis, the subneural gland as a proliferation from its ventral wall. 

 The pericardium appears as a proliferation from the right side of 

 the inner vesicle in the posterior region. The mass becomes hollowed 

 out to form a vesicle, one side of which invaginates to form the heart. 

 The sex organs develop by proliferation from the left- side of the 

 inner vesicle, the single primordium acquiring a lumen and dividing 

 into two, an ovary and a testis. 



In the anterior region of the body (Fig 1). the peribranchial sacs 

 (l.p.s, r.p.s.) are cut off from the more central part of the inner 



vesicle (hr. s.) by ventro- 

 lateral folds which grow 

 upward and meet each 

 other, thus separating off 

 a median portion or atrium 

 as well as the lateral parts. 

 In the posterior region of 

 the body on the contrary, 

 these folds do not meet 

 each other, but grow up- 

 ward and meet the orig- 

 inal dorsal wall of the in- 

 ner vesicle. They thus cut 

 ^ , . ^ off two lateral sacs which 



I. — Section of bud, transverse to long axis of 



«., atrium; hi. s., blood sinus; be s branchial ^rc entirely independent 



endostyle ; ec, ectoderm; l.p.s. and r.p.s, left ^ 



and right peribranchial sacs, x 133. of Cacll Other at this IcVcl. 



Between these two sacs along the mid-dorsal line lies the terminal por- 

 tion of the intestine as it passes forward to open into the cloaca. Sec- 

 tions of adults show that these posterior extensions of the peri- 

 branchial sacs never unite dorsally. In early stages the left peri- 



^ Geo. Lefevre, Budding in Ecteinusridia. Anat. Anz. Bd. 13, No. 18. 1897. 



Fig. 

 latter, 

 sac 



