AND DEVELOPMENT OF PYROSOMA. 211 



plast or nucleus. The cells measure on an average 4o~o ott of an inch in diameter ; and 

 their processes become very fine before they are lost in the surrounding nearly homoge- 

 neous matrix. In two regions this general structure is departed from. At the cloacal 

 wall of the test, there lies immediately beneath the surface a thin film of reticulated 

 tissue, consisting of cells similar in their essential structure to those just described, but 

 set more closely, more granular, elongated, and united together by the coalescence of 

 their processes. Again, in a plane which would correspond Avith the peripharyngeal 

 ridges of the ascidiozooids, and therefore near the outer surface, the test exhibits a very 

 faint longitudinal striation, as if it were fibrillated. 



There is no distinct epidermic layer on either face of the test. The corpuscles are, as 

 usual, stained dark yellowish brown by iodine ; whUe the matrix yields, though weakly, 

 the characteristic reaction of cellulose. 



§ 3. The Agamogeiiesis by Gemmation of Pyrosoma giganteum. 



Throughout the whole extent of the ascidiarium, the number of ascidiozooids appears to 

 be undergoing a constant increase, by the development of buds from those which already 

 exist ; at least, I have not yet met with any adult ascidiozooids devoid of a more or less 

 advanced appendage of this kind. Gemmation always takes place from that part of the 

 middle of the haemal side of the body of the ascidiozooid, which lies opposite the bend of 

 the intestine and between the posterior extremity of the endostyle and the reproductive 

 organs. At first, therefore, the bud is situated near the posterior or cloacal end of the 

 body, and on the same side as the closed apex of the ascidiarium. 



Gemmation does not take place in Pyrosoma, as in so many of the lower animals (e. g. the 

 Hyclrozoa and Folyzoa, or Salioa and Clavelina among the Ascidians), by the outgrowth of 

 a process of the body-wall whose primarily wholly indifferent parietes become differen- 

 tiated into the organs of the bud, but, from the first, several components, derived from 

 as many distinct parts of the parental organism, are distinguishable in it, and each com- 

 ponent is the source of certain parts of the new being, and of these only. Thus the 

 body -wall or external tunic of the parent gives rise to the external tunic of the bud ; 

 while a process of the endostylic cone of the parent is evolved into the alimentary tract of 

 the bud, and the reproductive organs of the latter are furnished by a part of that tissue 

 whence the reproductive organs of the parent took their origin. 



PI. XXX. fig. 14 represents the condition of what I may term the gemmiparous region 

 of the body in a young ascidiozooid, in which no distinct trace of a bud is discernible ex- 

 ternally. The outer tunic, it will be observed, passes evenly backwards, and has the same 

 structure in the situation of the future bud as elsewhere. The endostyle is continued 

 upwards and backwards as a celkilar cord, which contains a cavity continuous with the 

 groove of the endostyle, is about rtoutli of an inch thick, and is rounded-off at its extre- 

 mity. From this a thin sheet of indifferent tissue is continued downwards and backwards, 

 so that its plane forms nearly a right angle with the direction of the end of the endostyle, 

 and suddenly thickens to -f^o^h. of an inch. After this it tapers off gradually to its 

 extremity, which lies free in the cavity of the blood-sinus, at a distance of TsTT'th of an 

 inch from the ovisac of the ascidiozooid, which is -^roth of an inch in diameter, and so far 



2f 2 



