484 TUNICATA. 



(1269.) The reproductive system in these humble forms of Mollusca 

 presents the utmost simplicity of parts, being composed of an ovarian 

 nidus, in which the germs of their progeny are elaborated, and a duct, 

 through which their expulsion is accomplished. 



(1270.) The researches of Milne-Edwards*, however, relative to this 

 part of their economy, tend to show that the structure of these creatures 

 is more complex than was previously supposed ; and in Amaroucium 

 Argus, one of the compound Ascidians, that indefatigable observer also 

 succeeded in recognizing the presence of a male apparatus. This con- 

 sists of a largely-developed testicular gland, that occupies almost all the 

 lower part of the post-abdomen, and communicates with the common 

 excretory cavity by means of a long filiform canal that was regarded 

 by Savigny as the oviduct. This gland is made up of a multitude 

 of whitish vesicles, which, at first sight, have much the appearance of 

 rudimentary eggs, but which, in reality, are found to swarm with sper- 

 matozoa, thus revealing the true nature of the organ. A similar ar- 

 rangement has since been detected in numerous other genera ; so that 

 its existence throughout the entire class is now no longer doubtful. 

 The ovarium is situated in close juxtaposition with the testes. In all 

 the Polyclinian group it is lodged in the post-abdomen, and posteriorly 

 is hardly distinguishable from the male apparatus, but towards the 

 upper part becomes easily recognizable in consequence of the size and 

 colour of the ova contained within it. The eggs, of which a few only 

 are developed at a time, as they issue from the ovigerous organ pass 

 immediately into the cloaca, or even become lodged in the lateral part 

 of the thoracic chamber, between the proper tunic of this cavity and the 

 branchial sac, where they remain for some time exposed to the influence 

 of the surrounding aerated medium. 



(1271.) Whilst the ova remain enclosed in the upper part of the 

 post-abdomen, they increase considerably in size and assume a spherical 

 form ; the yelk acquires a deep-yellow colour ; and the vesicle of Pur- 

 kinje, which was distinctly visible during the commencement of their 

 development, disappears, and is replaced by a cloudy spot, which appears 

 to be the blastoderm, or proligerous layer, from which proceeds the 

 embryo of the young Ascidian. 



(1272.) During incubation the egg acquires more of an oval form, 

 the vitelline mass seems to contract, and its surface, growing denser, 

 appears to become organized into a membrane which is distinct from 

 the more deeply-seated substance of the yelk, and gradually the 

 whole becomes moulded into something like the shape of a tadpole 

 (fig. 245, A), the anterior extremity of which is surmounted with a 

 sort of tentacular apparatus ; and on the bursting of the egg, this 

 embryo, by means of its long tail, swims about in the water with con- 

 siderable vivacity ; after the lapse of a few hours, however, the little 

 * " Observations sur les Ascidies composees," Mem. de 1'Acad. torn, xviii. 



