428 



TUNICATA. 



now form a layer of large, swollen cells in contact with the ectodermal 

 capsule, while others form a granulated trabecular network traversing 

 the cavity of the placenta (Figs. 213 B, 214, x and z). The cavity of 

 the placenta, therefore, as LEUCKABT rightly pointed out (No. 98), 

 communicates with the blood-vascular-system of the parent, but never 

 with the body-cavity of the embryo, from which it is always divided 

 by the continuous ectodermal capsule of the placenta. Our own 

 researches have here led us to differ from SALENSKY, according to 

 whom the placental cavity appears as a part of the body-cavity of the 

 embryo. 



FIG. 214. Longitudinal sections through two later embryonic stages of /Salpa demo- 

 cratica-mucronata (after SALENSKY, somewhat altered), a, contracted outer lamella 

 of brood-sac ; b, blood-sinus within the placenta ; cl, atrium or peribrauchial cavity ; 

 d, rudiment of the alimentary canal ; e, epithelium of the atrial cavity of the parent ; 

 ec, ectodermal covering of the placenta ; el, elaeoblast ; h, rudiment of the heart ; 

 n, nervous system ; pc, pericardial vesicle ; ph, pharyngeal cavity ; t, so-called roof 

 of the placenta (basal plate) ; x, tissue of the placenta ; z, marginal cells of the 

 placental tissue. 



The pla.centa is concerned in the nourishment of the embryo. In 

 later stages it attains greater independence, its connection with the 

 embryo becoming constricted (Fig. 216). The point at which the 

 placenta is inserted lies on the ventral side of the embryo, between 

 the two endostyle-folds. When the embryo separates from the parent, 

 the placenta remains hanging to it ; in the free-swimming solitary 

 forms, a vestige of this organ is long visible as a small stalked body 

 enclosed in the mantle-substance (LEUCKABT). The wound formed 

 in the wall of the respiratory (cavity of the parent when the embryo 

 detaches itself is, according to LEUCKABT, closed by the remains of 





