34 Prof. T. H. Huxley on the Development o/Pyrosoma. 



to explain in detail the further steps of the metamorphosis of 

 the young Pyrosoma, the structure of the adult, or the agamo- 

 genetic development of young zooids in it ; but I may mention 

 that the 'cap' is the rudiment of the cloaca, a structure at first 

 totally distinct from the four Ascidiites (as the members of com- 

 pound Ascidians might be termed, in harmony with Polypite, 

 Somite, &c.), and homologically equivalent to one of them. 

 The four Ascidiites, connected by their gradually-lengthening 

 intermediate necks of blastoderm, change their positions so as to 

 become, as it were, wound half round the base of the rudiment of 

 the cloaca, with which they become gradually united, without at 

 first in any way opening into it. Enlarging faster than the 

 rudiment of the cloaca, they next completely encircle it, and so 

 extend, on each side, beyond it and the ovisac which it caps, as 

 almost to hide both these structures. The different organs make 

 their appearance, and the embryo attains a size of T n ^th of an 

 inch, or thereabouts. I have met with no embryo larger than 

 this, and I doubt if such are to be found contained in the parent 

 organism, as they fill the cloacal chamber of the atrium so com- 

 pletely that there seems to be no room for any further enlarge- 

 ment. Indeed, it is difficult to understand how they get out, 

 so disproportioned are they to the diameter of the cloacal aper- 

 ture of the zooid. 



The largest embryos exhibit neither rudimentary buds nor 

 reproductive organs, and each has two short canals passing from 

 the inner or hinder end of its neural surface towards the aper- 

 ture of the general cloaca. The latter lies altogether above the 

 primitive rudiment of the cloaca j and into it the cloacal apertures 

 of the four Ascidiites open. The primitive neck-like portion of 

 blastoderm connecting the zooid nearest the rudiment of the 

 cloaca with it is now converted into a long canal, which debouches 

 just in front of the ganglion, while from the opposite side and 

 end of the zooid, close to the end of the endostyle, a similar 

 slender tube passes to the region in front of the ganglion in the 

 next zooid. The other zooids are connected in the same way ; so 

 that the four Ascidiites and the rudimentary cloaca are tied to- 

 gether head and tail, like horses going to a fair, the 'tail* of 

 the last zooid, as of the last horse, being unfettered. 



By the observations of Miiller on Entoconcha and of Gegenbaur 

 on Sagitta, the question as to the fate of the germinal vesicle 

 after impregnation, or, rather, after yelk- division, has been re- 

 opened ; and the observations I have detailed tend to show that 

 in Pyrosoma, as in Entoconcha and Sayitta, the embryo-cells 

 arc the lineal descendants of the germinal vesicle, and that the 

 doctrine of Reinak and Virchow is applicable to the early stages 

 of development. 



