KUNGL. SV. VET. AKADEMIENS HANDLINGAR. KAND 63. N:0 9. 



17 



Stage V (Pl. 1, figs. 10—11. Text-fig. 8). A full-grown individual with deve- 

 loping embryos is shown in fig. 11. The testes are fully developed, the spermatic 

 lobes, visible on the outer side of the gland, are numerous and svvollen; the inner, 

 i. e. the mesial, side shows no lobation, as appears from fig. 10 on which there is also 

 to be distinguished the short vas deferens, which opens into the peribranchial cavity. 



The individual described has three embryos on the left side, which project 

 deeply inward and encroach upon the space of the peribranchial cavity. They are all 

 in a late stage of division. At the base each of them is enclosed within a cup-shaped 

 fold formed of the outer peribranchial wall, as described above. The fold is of varying 

 height i. e. the cup-shaped structure varies in depth, and hence in some zooids the 

 embryos are almost entirely enveloped in it, while in others, as in the individual 

 represcnted on fig. 11, the base only of the embryos is encircled by the fold. It 

 ought to be pointed out, however, that the structures are never in the form of well 

 marked-off globular sacs as is the case for instance in Metrocarpa leachi. 



The cup-shaped form is plainly visible from Pl. 1, fig. 10, in which the struc- 

 tures are shown from the inside, the embryos having been removed. The bottom 

 of the cup is solid; each cup is provided with a foot as it were, formed by the 

 constriction of the surrounding peribranchial wall (ef. Pl. 1, fig. 11. Text-fig. 8). In 

 the zooid here figured the structures are three in nu m ber on each side. The number 

 varies in conformity with that of the embryos, each embryo being attached by a 

 well-developed cup-shaped fold; but they are always situated close to one another, 

 and when numerous form a group on each side of the body. 



In text-fig. 8 there is shown a section made through an embryo, in a late stage 

 of division, attached to the peribranchial wall. The development of the placenta-like 

 structure may be understood from a comparison of this with the stage represented 

 in text-fig. 7. When the egg has thrown off the outer 

 follicle and passed into the peribranchial cavity, its folli- 

 cular epithelium — as well as that of the oviduet — grows 

 shorter, owing to great contraction, for the epithelium of 

 the outer follicle has been exposed to severe tension owing 

 to the growth of the ovum. The consequence of that con- 

 traction is, that the opening into the peribranchial cavity 

 soon becomes closed. It is thus to be supposed that the 

 whole cup-shaped structure including the bottom of it, 

 is formed of the outer wall of the peribranchial sac. When 

 the opening into the peribranchial cavity is closed, i. e. 

 when the bottom of the cupshaped structure has been 

 fully formed, the outer follicle, which may be considered as analogous to the corpus 

 luteum in other Ascidians, is separated from the peribranchial wall and commences 

 to degenerate. The embryo remains attached to the peribranchial wall, until it has 

 undergone its full larval development. 



The results of Della Valle's investigation of the organs in question do not 

 agree with what has been stated about the subject in the present note. According 



K. Sv. Vet. Akad. Handl. Band 63. X:o 9. 3 



Text-fig 8. Botryllus schlosseri Pallas. 

 Section through a cup-shaped fold with 

 an embryo. XI K), ef. Cupshaped fold. 

 ec Ectoderm. em. Embryo, p Outer 

 wall of the peribranchial cavity. 

 t. Testis. 



