16 AUGUSTA ÄRNBÄCK-CHRISTIE-LINDE, NORTHERN AND ARCTIC INVERTEBRATES. 



loped as are also the inner-organ systems. The testes have the same position as in stage 

 II, being situated towards the ventral side and behind the female organs; they have 

 increased in size and the lobes of the outer surface are well developed and numerous 

 (ef. Pl. 1, fig. 9). 



The female organs of this individual are in an interesting stage. No ovaries 

 containing oviduets and eggs are visible. In the place they occupied three thick-walled 

 vesicles are to be observed, often only groups of cells in degeneration, each representing 

 the outer follicle throvvn off by theovumi. e. »the corpusluteum», which, in this individual, 

 is still eonnected with the outer peribranchial wall. This connection is more distinctly 

 shown in fig. 8, where the egg has just passed out of the follicle. On the dorsal side 

 and in front of the last-mentioned struetures, the eggs may be observed, which now 

 lie in the peribranchial cavity and are in early stages of division. The embryos are not 

 free, b ut are attached to the wall of the peribranchial cavity each of them by a cup- 

 shaped fold or placenta-like strueture, which is found more distinctly developed in the 

 following stage. 



The relation of the outer follicle to the cup-shaped fold is seen in text-fig. 7, 

 which displays a transverse section made through this strueture in the individual 

 illustrated in Pl. 1, fig. 8. The egg has just thrown off the outer follicle and has 

 entered the peribranchial sac through the oviduet. The fertilization has taken place 

 and the division has commenced. The lumen of the follicle is still in communication 

 with the peribranchial cavity. The epithelium of the outer follicle, as well as that of 

 the oviduet, is highly contracted and is still eonnected with that of the outer peribran- 

 chial wall by a constricted neck. Soon, however, the empty follicle is cut off from the 

 peribranchial wall and begins to degenerate; a remnant in the form of an irregular 

 cell-group may persist for some time, but it soon disintegrates and disappears. 



This stage reminds one of a stage in the development of Metrocarpa leachi, descri- 

 bed above and illustrated in text-fig. 3. On making a comparison of the two it may, 

 at first sight, appear as though a certain resemblance was to be perceived, but on 

 closer examination the differences are seen to be of the highest importance. The 

 resemblance seems to bear reference to the physiological act which is illustrated rather 

 than to any points of agreement in the anatomical strueture. In both figures the egg has 

 just been extruded out of the outer follicle, the fertilization has taken place and the divi- 

 sion has commenced. The empty follicle still persists in the mesoderm, and its epithelium, 

 though highly contracted, is eonnected with that of the cavity into which it opens. 

 The epithelium of the oviduet too is highly contracted, and hence the duet has become 

 rather short. Apart from this, there are hardly any other points of agreement. In Metro- 

 carpa leachi the ovum passes into a brood-pouch, a uterus-like strueture, forming a 

 diverticulum of the peribranchial cavity, which develops into a globular sac that is en- 

 closed by the mantle and is situated on the outside of the posterior end of the body- 

 wall, projecting into the common test. In Botryllus schlosseri the ovum enters the peri- 

 branchial cavity and is attached to its wall by a cup-shaped fold or placenta-like strue- 

 ture so to say, formed by the peribranchial epithelium, and there the embryo remains 

 until it has attained its full larval development, as will be shown in the following pages. 





