10 Emlryology of Saljpa. By W. K. Brooks. 
other fall-grown chains, are drawn into the branchial sacs of the 
immature zooids which contain the eggs, and penetrate into the 
interior of the gubernaculum. 
Upon impregnation the germinative vesicle disappears; the 
gubernaculum becomes irregularly swollen and shortened, thus 
drawing the egg down into the brood-sac, which is formed by an 
involution of the branchial sac of the nurse (Fig. II.). The egg, 
nourished by the blood which bathes it, rapidly increases in size, 
and undergoes a process of total segmentation, as the result of 
which two portions are formed ; a finely segmented " germ yolk," 
and a less completely segmented " food yolk." (Fig. Y.) 
The latter becomes enveloped by the former through a process 
of invagination, forming a true " gastrula " or " invaginate planula," 
the opening of which, the " orifice of Kusconi," persists and forms • 
the orifice of the placenta. (Figs. YI., YII., YIIL, /.) 
The embryo, still growing rapidly, becomes divided into two 
portions by a constriction (Fig. YII.) ; the portion nearest the 
point of attachment to the brood-sac forms the embryo proper, and 
the remaining portion that part of the placenta which is to be in 
communication with the sinus system of the foetus. (Fig. YII.) 
Within this portion there is a cup-shaped cavity, part of the 
original "cavity of Kusconi," which is in direct communication 
with the sinus system of the nurse, and thus forms the second or 
inner chamber of the placenta. This soon becomes divided up into 
a great number of irregular intercommunicating lacunae, which are 
produced by the growth of a structure resembling a stump with its 
roots, and which seems to be formed directly from the blood of the 
nurse, by the aggregation and fusion of the blood-corpuscles. 
The subsequent development of the foetus, which is the young 
of the solitary Salpa, is substantially as it has been described by 
Sars, Krohn, Yogt, Huxley, Leuckart, and others, and I have been 
able to add little to what is known upon the subject. 
The atrium of Salpa has been supposed to lack those lateral 
portions which, in most Tunicates, lie upon the sides of the 
branchial sac and are called the lateral atria ; but at an early stage 
these seem to be present, as well as the mid-atrium, but the cavities 
of the lateral atria never become connected with that of the 
branchial sac by the formation of branchial slits ; and at a very early 
period of development the walls of each lateral atrium unite, thus 
obliterating the cavity, and giving rise to a broad layer of tissue 
upon each side of the body, between the branchial sac and the so- 
called "muscular tunic," the "outer tunic" of Huxley.* Kows of 
transverse slits soon appear in these layers, which thus become 
divided to form the muscular bands, which latter subsequently 
* This " outer tunic must not be confounded with the " cellulose test " of 
Huxley, which covers it. 
