SALPIDAE FORMS WITHOUT COVERING FOLDS. 427 



a tube, the inner cells being transformed into blood-corpuscles which 

 pass into the blood. 



Before passing on to describe the other changes that take place in 

 the embryo we must dwell for a moment on the degeneration of the 

 brood-sac and the development of the placenta. After the inner 

 lamella of the brood-sac has degenerated as described above, the 

 embryo remains surrounded solely by the very thin epithelium of the 

 outer lamella (Fig. 212 .#, a), which consists of a differentiated part 

 of the atrial epithelium of the parent. This outer lamella is unable 

 to keep pace with the further increase in size of the embryo ; it 

 becomes ruptured at the point at which the aperture of the oviduct 

 was originally situated and shrinks downwards over the embryo (Fig. 

 213). In consequence of this contraction of the outer lamella the 

 embryo, which originally lay in the follicle (Fig. 211 A), and then 

 shifted forward into the dilated oviduct (Fig. 211 B), protrudes 

 into the atrial cavity of the parent, in which from this time it lies 

 freely. 



We 'have already seen (p. 424) that a compact cell-mass is attached 

 to the lower surface of the embryo (Fig. 212, />) ; this, which 

 represents the first rudiment of the placenta, is derived from the 

 transformed cell-material of the egg-follicle. The outer lamella of the 

 brood-sac now shrinks completely back over this cell-mass, and finally, 

 as a constricted funnel-like annulus, surrounds and strengthens the 

 connection between the rudiment of the placenta and the parent (Fig. 

 214, a). The placenta-rudiment would lie exposed, after the with- 

 drawal of the outer lamella, were it not covered by a thin ectodermal 

 layer of the embryo (Fig. 214, ec), which develops as the brood-sac 

 is withdrawn. Through this circumcrescence of the placenta by an 

 ectodermal lamella, which was not observed by SALENSKY, but of 

 which we were able clearly to convince ourselves, the placenta is 

 incorporated in the embryo and then appears enclosed in a capsule 

 derived from the ectoderm of the embryo. The lateral walls of this 

 capsule are formed by the thin lamella just mentioned ; its upper 

 wall or the so-called roof (Fig. 213, t\ on the contrary, is yielded by 

 the thick ectoderm -layer, the origin of which was traced above (p. 425). 

 [This is of follicular origin according to KOROTNEFF (No. XXa.).] On 

 its under side the ectodermal capsule of the placenta possesses an 

 aperture through which the placental cavity communicates with the 

 blood-vascular system of the parent. The placental cavity arises in 

 the form of gaps or clefts in the placental tissue, which thus assumes 

 a loose structure. Some of the cells of this originally compact tissue 



