69 



whichj measured from the stand-point of the original develop- 

 ment, is retrogressive. This condition is introduced by a 

 period of rest after the free-swimming larva has attached itself 

 in one of the manifold ways which have been described. 

 Whilst the chorda, the original muscles, and the upper skin 

 of the tail shrink together and degenerate into a single heap, 

 the nervous system takes part in the retrograde change, but 

 continues longer of the same form which it before possessed, 

 although continually decreasing in volume. As long as a 

 remnant of the tail is to be seen a thread-like cord of the 

 nervous system is to be seen stretching into it, which shows 

 the former relationship between this and the tail muscles. 

 The respiratory pharynx does not make rapid jDrogress, 

 for the gill sac was already separated from the intestine in 

 the free- swimming larva, and it had besides already developed 

 the foundations of the ciliated ridges without, however, 

 shoAving the cilia. 



During this condition of rest, for it is characterised by the 

 absence of any movement, the heart is formed, which con- 

 sists of a group of those small round cells spoken of above as 

 occurring inside the cavity of the body, whilst the rest, de- 

 creasing in numbers, become amoeboid cells, which are set 

 in motion by the first pulsations of the heart. They are the 

 blood-globules or rather lymph-globules, the cavity of the 

 body being a lymph sinus. With the commencing action of 

 the heart the condition changes afresh ; a true chrysalis stage 

 is terminated, the gill sac widens, the slits of its walls become 

 edged with cilia, the inhalent and exhalent orifices break 

 through the mantle, and with these changes the outward 

 form of the Ascidia appears distinctly. 



So much for the present concerning these things. I hope 

 during the vacation to send more minute particulars to the 

 " Archiv.' 



