464 



TUNICATA. 



genital rudiments can also be distinguished at an unusually early 

 stage.* The above is the case in the buds of Distaplia (Kowa- 

 levsky's Didemniwm styliferum), which, as free bodies detached from 

 the stolon, are found scattered in the cellulose substance of the colony. 

 In the genital strand (Fig. 235, g) of the youngest of these bads, 

 several young egg-cells can always be recognised. These buds, 

 however, are capable of multiplying by fission (Fig. 235 B), and in 

 this case the eggs become distributed so that one occurs in each 

 portion of the original bud. Kowalevsky is, therefore, inclined to 

 regard the buds which form first as stolons which have separated 



.— £C 



■au 



/- 



Fig. 235. — A, younger, B, older stage of development of Distaplia stylifera (after 

 Kowalbvskt). In B, the bud is dividing into two. ec, ectoderm ; en, entoderm ; 

 g, genital strand ; ms, mesoderm. 



from the Ascidiozooids, the parts resulting from fission alone repre- 

 senting the true buds, and Lahille has recently adopted this view. 



The entoderm-sac is the seat of the most important transformations 

 through which the bud develops into the young Ascidian. This sac 

 increases in size and its anterior margin becomes trilobed (Fig. 236). 

 The middle lobe must be regarded as the rudiment of the branchial 

 sac (pharynx), while the two lateral parts represent the rudiments of 

 the peribranchial sacs. These sacs are, therefore, in the bud of 

 Distaplia, distinctly entodermal in their origin. 



The peribranchial sacs grow round the sides of the middle ves- 

 icle and, at the same time, a process grows from each sac towards 



* [In the Didemnidae (Pizon, No. XXVTa.) the sexual cord is continued 

 from the parent into the bud. — Ed.] 



