no 



BIOLOGICAL LECTURES. 



It is when we turn to more special evidence, however, that 

 we discover how much caution is necessary in our treatment 

 of the germ-layers. The most striking of this evidence is 

 afforded by the extraordinary contradiction between the egg- 

 development and bud-development in certain animals, of which 

 I select the Tunicata as the best known example and one 

 which, through the courtesy of Dr. Hjort, I have myself had 

 an opportunity to examine critically. In the egg-development 

 of Tunicata, in all known cases, the atrial chamber is derived 

 as a pair of ectoblastic pouches invaginated from the exterior, 

 and the nerve-ganglion is as usual derived from the dorsal 

 ectoblast. In the bud-development the history is totally differ- 

 ent. In all cases the bud arises a two-layered vesicle of which 



DIAGRAM I. 



Very young diblastic tunicate bud. The outer layer always arises from the 

 parental ectoderm. The inner layer arises from the entoderm (epicardium or 

 pharyngeal wall) in Clavelina, Perophora, Didemnum, etc., from the ectoderm 

 (atrial wall) in Botryllus. 



the outer wall is continuous with the parental ectoblast. The 

 inner wall shows a surprising contrast in different forms. In 

 one series, represented by Perophora, Didemnum, Clavelina, 

 and some others, the inner vesicle is derived from the parental 

 entoderm, viz., from the wall of the branchial sac. In Botryllus, 

 on the contrary, there is not the least doubt that it arises from 



