8 AUGUSTA ÄRNBÄCK - 0HRISTIE-L1NDE, NORTHERN AND ARCTIC 1NVERTEBRATES. 



011 the figure. In the mesoderm there lies a mature egg ready to pass into the brood- 

 pouch. It lies pressed close to the body-wall, which is much distended. Both the 

 outer and the inner follicular epithelia are well differentiated — in the drawings they 

 are represented by two lines — even the test cells are easily distinguished. The oviduct 

 is to be seen between the brood-pouch and the egg; it is in the form of a thin-walled 

 rounded vesicle, compressed lengthways. It projects from the outer envelope of the 

 egg and lies close to the deep wall of the brood-pouch (ef. text-fig. 2). The figure 

 thus illustrates a strueture composed of follicle, oviduct and one egg, i. e. a strueture 

 analogous to an ovary. And if we consider that the egg — though only one — has 

 been formed and developed within it, it may be designated a one-egg ovary. Whether 

 the female organ of Metrocarpa consists of more than one such ovary on each side 

 cannot for the present be decided. Only one. however, develops on each side at the 

 same time. One-egg ovaries are not knovvn in higher Ascidians. They have been 

 observed and described in certain Thaliacea, viz. in the Pyrosomidce and the Saljndce, and 

 their occurrence in the Botryllidce may be considered as an interesting point of agree- 

 ment between groups so remotely allied as the three last-mentioned. 



As has been pointed out above, the stage just described is represented by a 

 full-grown zooid, in which the presence of fully developed male and female organs 

 is beyond all doubt. The statement of previous investigators to the effect that 

 ovaries are found in the young buds only and testes in full-grown individuals only, 

 is consequently not accurate. 



Stage IV (Pl. 1, fig. 3. Text-figs. 3—4). The zooid illustrated in fig. 3 is 

 full-grown, of about the same length as in the above stage. The siphons are open 



and well developed, the atrial one is broad and wide, the 

 atrial languet elongated. The testes are in the same 

 condition as in stage III, though of still larger size; 

 the lobes seem more voluminous and swollen. 



In the female organs a great difference between the 

 two stages is manifest. The brood-pouches are no longer 

 empty vesicles, they each enclose an egg or rather an embryo 

 Text fig. 3. Metrocarpaieachi Savxony in process of development. The connection between the 

 n. gen. Section tiuough the female brood-pouch and the peribranchial cavity is represented 



organs. XI 10. bp. Brood-pouch. ec L * ' . 



Ectoderm. em. Embryo. /. Outer by a short and well marked-olt canal, easily distmguishea 



follicle, thrown off. r> Outer wall of • ». n x 1 1 -\ ,i 11 j j „ 



the peribranchial cavity. m lig. 3. In the mesoderm another small rounded strueture 



is seen, which occupies the same position as the ovum at 

 an earlier stage. The said strueture is connected with the bottom of the brood-pouch. 

 Sections show that this is the empty outer follicle, thrown off by the ovum before 

 passing into the brood-pouch. This follicle may thus be considered as analogous to 

 the corpus luteum of higher animals. 



Text-fig. 3 shows a transverse section made through the brood-pouch and the 

 outer follicle in the corresponding stage. The egg has only just thrown off the outer 

 envelope and passed through the oviduct into the brood-pouch, where fertilization 

 has already taken place; the division has commenced. The embryo is enclosed in 



