SALPIDAE FORMS WITH COVERING FOLDS. 435 



by TODAKO the plarr nt <il iiifiii/>i-<iii< <>r ; /ermoblastica [the supporting 

 ring of the placenta (BROOKS)] . 



A differentiation somewhat resembling that just described in the 

 outer lamella is also found in the inner lamella (splanchnic layer of 

 the follicle) which represents the transformed epithelium of the ovi- 

 duct and follicle. The lower half of the lamella (Fig. 217 B, d), in 

 this case, becomes connected with the thickened epithelium of the 

 at rial cavity known as the placental membrane or supporting ring 

 of the placenta (e), and forms the roof of the placenta (Fig. 218 A, dp), 

 in the centre of which the " blood -bud " is attached. The placenta 

 is thus hollow, its lateral walls (supporting ring, Fig. 218 A, mp) 

 being yielded by the epithelial prominence of the atrium, and its 

 roof (dp) by the inner lamella of the brood -sac, i.e., by the follicle 

 (placental portion of the follicle). The " blood-bud " (bk) hangs 

 from the roof into the cavity which is part of one of the blood- 

 channels of the parent. SALENSKY distinguishes in this cavity two 

 communicating sinuses, an afferent and an efferent sinus, and, be- 

 tween these two, a third vascular space round the " blood-bud" (Fig. 

 :M7 A, />/'), the relations and significance of which are unknown. 



The placenta, which, from the first, is a greatly swollen structure, 

 now becomes constricted at its base (Fig. 218), and thus forms a 

 stalked structure on the upper surface of which the embryo rests. 

 It becomes saddle-shaped later, the parts lying at the sides of the 

 embryo growing upward. This is why,, in a horizontal section (Fig. 

 219) only the lateral parts of the placenta (p) are seen cut through. 

 The actual relations of this organ are still very obscure. 



While, in this way, the primary brood-sac undergoes essential 

 alteration, a circular fold of the atrial epithelium grows up from 

 the base of the epithelial prominence (Figs. 210 B, and 217, fh) 

 and completely overgrows the placenta and the embryo (Fig. 218, /), 

 and thus forms a new secondary brood-sac (embryo-sac) such as is 

 not found in S. democratica-mucronata. This is known as the cover- 

 ing fold or amnion. It continues to grow upwards as a circular fold 

 round the embryo, over which, however, it never fuses, but remains 

 separated by a variable aperture through which the embryo eventu- 

 ally passes out into the atrium. 



There are, in the different species, characteristic variations in the form of 



this fold. In S. (ifricnn(i-m(i.rim(i, the aperture is elongate and the margins 



of the fold project and take the form of a semicircular crest ; this is seen in 



--section in Fig. 221, c; in S. fusiformis, this crest is abruptly truncated. 



In iS. plnnata and S. punctata, on the contrary, such a crest is wanting. 



