434 TUNICATA. 



which must be regarded as the direct descendants of the blastomeres. 

 These seem to multiply little, but, up to the stage at which the 

 organs begin to develop, continue to be seen in the embryo (Fig. 219 

 A, b). The significance of these parts and their further fate are 

 obscure. The numerous small cells which form the greater part of 

 the embryo are all regarded by SALENSKY as immigrated follicle-cells, 

 but we think ourselves justified in assuming, with TODARO, that they 

 are partly derived from the blastomeres and are only partly immi- 

 grated follicle-cells. The latter cells disintegrate, while the former 

 take an essential share in the formation of the embryo (p. 420 and 

 footnote, p. 421).* 



The embryo lies in a sac (Fig. 217 A, fw) which is derived from the 

 union of the follicle and the dilated oviduct. The wall of this sac has 

 been described in S. democratica-mucronata as the inner lamella of the 

 brood-sac. At the posterior end of this sac a thickening is soon seen 

 in the forms now under consideration ; this becomes more and more 

 distinct (Fig. 217 B, bk) t and is to be traced back to the transformed 

 wall of the follicle. While, in S. democratica-mucronata, this thicken- 

 ing forms the rudiment for the whole of the placenta, in S. pinnata 

 and related forms it represents a comparatively small part of that 

 organ, which does not develop further but, in later stages, breaks up 

 into its elements and mingles with the blood of the parent (or of 

 the embryo ?). This part has been called by TODARO the blood-bud 

 (bottom ematogene, Figs. 217-5, and 218, bk). 



The embryo almost entirely fills the cavity of the brood-sac (Fig. 

 217, /). On one side, it appears to be continuous with the inner 

 lamella of the sac. According to SALENSKY, this side represents the 

 later haemal side of the body, so that we are able, even at this stage, 

 to orient the embryo. [According to BROOKS (No. I.), this, on the 

 contrary, marks the middle dorsal line of the embryo.] 



The outer lamella of the brood-sac (Fig. 217, e and e) is derived 

 from the thickened part of the atrial epithelium of the parent, which 

 is pushed out into the atrium by the growth of the embryo beneath 

 it, and which is called by SALENSKY the epithelial prominence (p. 417). 

 Two parts can soon be distinguished in it. The upper part (e), which 

 covers the greater part of the embryo, consists of rather small low 

 cells, while the lower part (e) is composed of deep columnar cells. 

 This latter yields later the lateral walls of the placenta, and is called 



* [It is now generally believed that the embryo at the end of cleavage con- 

 sists of two kinds of blastomeres, i.e., large and small, the latter being in 

 appearance indistinguishable from the kalymmocytes. ED.] 



