876 SUMMAEY OF CUERENT RESEAECHES RELATING TO 



mode of development of the nervous system is very closely similar 

 to that of AmpMoxus ; the tube is for some time open at its anterior 

 end, and in communication with the outer world. The mesoderm 

 has a paired endodermal origin. 



The fact that some of the organs begin to appear very early can, 

 in Seeliger's opinion, only be explained by supposing that the Ascidian 

 is derived from a more highly organized ancestral form, which was 

 provided with a head and trunk. 



The fourth period of development is distinguished as that of 

 histological differentiation ; in the fifth we have the free-swimming 

 larva. That of Clavelina is more highly organized than the larva of 

 simple Ascidians ; the enteric tract is more complicated, and all the 

 parts of the adult are to be recognized ; the peribranchial cavity is, 

 with rare exceptions, provided only with a single egestive orifice, and 

 there are only two rows of clefts on either side ; these, however, soon 

 increase in number, and the dorsal exchange their rounded for an 

 elongated, elliptical form. Part, at least, of the persistent muscula- 

 ture is formed of mesodermal cells, which had previously no function ; 

 and it is certain that the two kinds of muscles — the epithelial mus- 

 culature of the tail, and the mesenchymatous muscles of the anterior 

 portion of the body — which had been sharply separated by Hertwig, 

 both arise from a common rudiment of mesoderm. At the end of 

 this larval period all the organs consist of one layer, and their mode 

 of origin may be summed up in the annexed table. 



After a period of active life the larva begins to attach itself by 

 the papillge at the most anterior end of the stolon, so that the mouth 

 of the animal is now turned downwards. Soon the primary axis 

 begins to run parallel to the ground, and the mouth finally comes- to 

 lie superiorly ; the communication between the cavities of the body 

 and of the stolon becomes considerably narrower, and the stolon itself 

 presents the greatest variations. The process of the absorption of 

 the larval tail begins with the protrusion of the inner layers from 

 the ectodermal tegumentary tube; the latter grows shorter, and its 

 cells thicken. While the inner layers become spirally rolled up the 

 few cells are set loose, as free mesodermal cells, into the circulation ; 

 the rolling up is effected gradually. What cells remain form an 

 amorphous brown mass which, in consequence of the growth of the 

 body, appear to pass forwards, and lie in the anterior part of the 

 stomach. Finally, this also is dissolved, and no indication of the 

 caudal segment is left. The whole of the nervous apparatus breaks 

 up into its cellular elements. The growth of the young is princi- 

 pally effected in the direction of the long axis of the body, and in 

 this way the plump foro'-body of the larva becomes converted into 

 the cylindrical body of the adult Clavelina. No indication remains, 

 after the break up of the tissues of the tail, which would induce us 

 to regard the Ascidian as one of the Coelomata ; the large primary 

 ccelom is filled with free cells. The ciliated pit which early had the 

 appearance of a canal, wide anteriorly and gradually growing narrower, 

 has, later on, the anterior separated by a groove from the posterior 

 part ; the former widens considerably, and the canal disappears from 



