SALPIDAE POEMS WITHOUT COVKKINd FOLDS. 127 



,i tube, the inner cells bean; 1 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 



lnbrvo remains surrounded solely by the very thin epithelium of the 



>uter lamella (Fig. 212 B, a), which consists of a differentiated part 



)f 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 .1), and then 



shifted forward into the dilated oviduct (Fig. 211 />'), 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, ji) : 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 overthis 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, '()• 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 is withdrawn.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, f), 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 Korotnefk ( No. XXa.).] < hi 

 its underside 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 originallv compact tissue 



