18 MOLGULA. 



resulting from the atrophy of the tail are employed in the nutrition 

 of the mesoblastic structures of the trunk. 



The body cavity in the adult is well developed in the region of 

 the intestine, where it forms a wide cavity lined by an epithelioid 

 mesoblastic layer. In the region of the branchial sack it is reduced 

 to the vascular channels in the walls of the sack. 



Kowalevsky believes the body cavity to be the original segmenta- 

 tion cavity, but this view can hardly be regarded as admissible 

 in the present state of our knowledge. In some other Ascidian 

 types a few more facts about the mesoblast will be alluded to. 



With the above changes the retrogressive metamorphosis is com- 

 pleted ; and it only remains to notice the change in position under- 

 gone in the attainment of the adult state. The region by which the 

 larva is attached grows into a long process (fig. 10 B), and at the same 

 time the part carrying the mouth is bent upwards so as to be re- 

 moved nearly as far as possible from the point of attachment. By this 

 means the condition in the adult (fig. 11) is gradually brought about; 

 the original dorsal surface with the oral and atrial openings becoming 

 the termination of the long axis of the body, and the nervous system 

 being placed between the two openings. 



The genus Molgula i)resents a remarkable exception amongst the simple 

 Ascidians in that, in some if not all the species belonging to it, develop- 

 ment takes place (Lacaze Duthiers 29 and 30, Kupffer 28) quite directly 

 and without larval metamorphosis. 



The ova are laid either singly or adhering together, and are very 

 opaque. The segmentation (Lacaze Duthiers) commiences by the formation 

 of four equal spheres, after which a number of small clear spheres are 

 formed which envelope the large spheres. The latter give rise to a 

 closed enteric sack, and probably also to a mass of cells situated on the 

 ventral side, which appear to be mesoblastic. The epiblast is constituted 

 of a single layer of cells which completely envelopes the enteric sack and 

 the mesoblast. 



While the ovum is still within the chorion five peculiar processes of 

 epiblast grow out; four of which usually lie in the same sectional plane 

 of the embryo. They are contractile and contain prolongations of the body 

 cavity. Their relative size is very variable. 



The nervous system is formed on the dorsal side of the embryo before the 

 above projections make their appearance, but, though it seems probable that 

 it originates in the same manner as in the more normal forms, its develop- 

 ment has not been worked out. As soon as it is formed it consists of a 

 nervous ganglion similar to that usually found in the adult. The history 

 of the ma.-is of mesoblast cells has been inadequately followed, but it con- 

 tinuously disappears as the heai't, excretory organs, muscles, etc. become 

 formed. So far as can be determined from Kupffer's descriptions the 

 body cavity is jirimitively i)arenchymatous — an indication of an abbrevi- 

 ated development — and does not arise as a definite split in the mesoblast. 



The primitive enteric cavity becomes converted into the branchial sack, 

 and from its dorsal and posterior corner the oesophagus, stomach and 

 intestine grow ont as in, the normal forms. The mouth is formed by the 



