AND DEVELOPMENT OF PYROSOMA. 211 
plast or nucleus. The cells measure on an average 4-oWth 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 eloacal 
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 with 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 ; while the matrix yields, though weakly, 
the characteristic reaction of cellulose. 
§ 3. The Agamogenesis by Gemmation of Pyrosoma giganteum. 
Throughout the whole extent of the ascidiarium, the number of ascidiozooids app.ars 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 hamial 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 eloacal 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 
Hydrozoa and Polyzoa, or Salpa and ClaveUna 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 ttssuo 
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 cellular cord, which contains a cavity continuous with the 
groove of the endostyle, is about ^th 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 nearlv a right angle with the direction of the end of the endostyle, 
and suddenly thickens to ^th 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 xfeth of an 
inch from the ovisac of the ascidiozooid, which is ^th of an inch in diameter, and so far 
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