AND DEVELOPMENT OF PYROSOMA. 221 



cellular in structure, but it is devoid of any structureless investment or membrana propria. 

 The contained ovum consists of a solid-looking, well-defined germinal spot oVb otb of an inch 

 in diameter, occupying the centre of a germinal vesicle -sw^h of an inch in diameter, with 

 a thin but well-defined wall, and perfectly clear contents. The yeUi is represented by a 

 small zone of structureless, yellowish substance, which invests the germinal vesicle, and, 

 on the one hand, passes into the waU of the ovisac, while, on the other, it is separated 

 from that wall by a narrow clear space. 



Second Stage. Ovisacs less than ^ou^^' of an inch in diameter and unimpregnated. 



The ovisac represented in fig. 2. exemplifies this condition very well. It has a dia- 

 meter of s^o^h of an inch. Its form is spheroidal, and it is produced on the side towards 

 the atrial wall of the blood sinus in which it lies, into a short subcylindrical diverticulmn, 

 which is directed forwards, and slightly towards the neural side of the ascidiozooid 

 in which it lies. This diverticulum, or rudimentary duct, is 4^tli of an inch in length, 

 and its slightly narrowed anterior extremity passes into the atrial tunic. At its opposite 

 end, where it becomes continuous with the ovisac, it measures grotli of an inch in dia- 

 meter. At this extremity, the cavity of the duct is in free communication with that of the 

 ovisac, but at a little more than half way towards the opposite end, or in other words 

 towards the atrium, the cavity ceases, the termination of the duct appearing to be a solid 

 cellular mass. In this condition, therefore, there would appear to be no communication 

 between the interior of the ovisac and the atrial cavity. 



The wall of the ovisac exhibits no distinct membrana propria, but is composed of a 

 single layer of flattened corpuscles, about 5-^50-th of an inch in diameter, imbedded in, and 

 connected together by, a structureless substance. The wall of the duct is similarly com- 

 posed, but its hsemal is much thicker than its neural wall. In the cavity of the duct 

 nothing save a clear fluid is discernible, and the same fluid seems to fill the interval 

 observable on one side, between the wall of the ovisac and tlie ovum. 



The latter consists of a very finely granular, spheroidal vitelline mass ^^tti of an inch 

 in diameter, within which lies a germinal vesicle (e^th) with perfectly clear contents, 

 inclosing an opake, spheroidal, germinal spot (rttVoth). The yelk is in close contact with 

 the inner side of the anterior wall of the ovisac — the germinal vesicle is close to its sur- 

 face at the same point, and the germinal spot is applied to the inner surface of the ante- 

 rior wall of the vesicle, so that it is as near as possible to the wall of the ovisac. 



I have not been able to discover a trace of a vitelline membrane in ova in this stage. 

 It may be doubtful whether the space between the wall of the ovisac and the ovum is a 

 natural or an artificial product. My observations upon the ovisacs of a fresh Fijro- 

 soma (PhU. Trans. 1851) lead me to adopt the latter hypothesis. 



Third Stage. Ovisacs under jj^jyth of cm inch in diameter and in process of impregnation. 



(PI. XXX. figs. 3, 4, and 8*.) 

 Of the two specimens in this stage which I have figured, the larger (fig. 3) is rather 

 the less advanced. Its duet is longer than the diameter of the ovisac, and is not only 



