30 Prof. T. H. Hiixlcy on the Development o/ Pyrosoma. 



gular compound embryos described by Savigny, but which I 

 had been unable to find in the specimen which formed the sub- 

 ject of my memoir on Pyrosoma* published in the * Philosophical 

 Transactions/ nearly nine years ago. Such embryos proved to 

 be exceedingly abundant ; and as might have been expected from 

 Savigny's known accuracy, his account of their structure turned 

 out to be perfectly correct, so far as it goes. The excellent state 

 of the specimen, however, led me to endeavour to trace out the 

 origin and mode of development of these bodies; and in so 

 doing I came upon facts of such an anomalous character, that 

 the inquiry assumed much larger proportions, and became in- 

 vested with a far wider interest, than I had anticipated. 



I must premise that the tissues of Pyrosoma, as of all 

 Ascidians, are excellently preserved by immersion in tolerably 

 strong spirit — so that the finest details, down even to the ends 

 of the cilia, are admirably exhibited ; while the cautious addition 

 of glycerine to sections of spirit-specimens renders them more 

 transparent, without, so far as I have perceived, doing them 

 any uijury. 



I will now proceed to describe the process of development 

 undergone by the impregnated ova. 



As I have pointed out in the memoir referred to above, each 

 zooid of a Pyrosoma contains a large testis, and a single ovum 

 enclosed within an ovisac but little larger than itself. Until the 

 ovisac attains a diameter of ^^th of an inch, or thereabouts, the 

 duct which connects the ovisac with the wall of the atrium, or 

 cloacal cavity of the zooid, is imperforate at its extremity, and its 

 walls, like those of the ovisac, are composed of a single layer of 

 cells without any limitary membrane or tunica propria. There are 

 no cilia either on the wall of the ovisac or on that of the duct. 

 The ovum consists of a large, spherical, clear germinal vesicle, 

 containing a spherical, more or less solid-looking germinal spot, 

 and surrounded by a yellowish, very finely granular yelk, which 

 is totally devoid of any vitelline membrane. The germinal 

 vesicle lies on one side of the yelk, and the spot on the same 

 side of the vesicle. 



In ovisacs of larger size (up to youth of an inch in diameter) 

 the duct is pervious throughout ; and, as it is open at its extre- 

 mity, it is in free communication with the atrium, and, through 

 that, with the cxteiior. The ovum still fills the ovisac, its yelk 

 being in the same condition as before, but larger, both absolutely 

 and in relation to the germinal vesicle. The latter has also 

 enlarged up to a diameter of about ^yijth of an inch, but it is 



* Observations on the Anatomy and Physiology of Salpa and Pyrosoma, 

 ' Philosophical Transactions,' 1861, p. 2, 



