22 MOLGULA. 



adult (fig. 11) is gradually brought about; the original dorsal 

 surface with the oral and atrial openings becoming the termina- 

 tion of the long axis of the body, and the nervous system being 

 placed between the two openings. 



The genus Molgula presents a remarkable exception amongst the simple 

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

 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) commences 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 

 development 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 mass of mesoblast cells has been inadequately followed, but it 

 continuously disappears as the heart, excretory organs, muscles, etc. become 

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

 cavity is primitively parenchymatous an indication of an abbreviated 

 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 out as in the normal forms. The mouth is formed by the 

 invagination of a disc-like thickening of the epidermis in front of the nervous 

 system on the dorsal side of the body ; and the atrial cavity arises behind 

 the nervous system by a similar process at a slightly later period. The gill 

 clefts opening into the atrial cavity are formed as in the type of simple 

 Ascidians described by Krohn. 



The embryo becomes hatched not long after the formation of the oral and 

 atrial openings, and the five epiblastic processes undergo atrophy. They 

 are not employed in the attachment of the adult. 



The larva when hatched agrees in most important points with the adult ; 

 and is without the characteristic provisional larval organs of ordinary 

 forms ; neither organs of special sense nor a tail becoming developed. It 

 has been suggested by Kupffer that the ventrally situated mesoblastic mass 



