440 



TUNICATA. 



boundary between the embryo and the placenta arises. In the stage 

 now being considered the body-cavity seems to be separated from the 

 placental cavity only by the lamella which was mentioned above as 

 the roof of the placenta (dp), but we do not know to what extent this 

 lamella participates in the formation of the embryo. 



Further development leads to the complete disappearance of the 

 primary body-cavity which becomes filled in the way mentioned above 

 by a mesenchyme (Fig. 222, wis), the elements of which give origin 

 not only to the connective tissue but also to the blood-corpuscles, the 



Fig. 221. — Transverse section through an embryo of Salpa africana-maxwia (after 

 Salenskt). a, epithelium of the respirators cavity of the parent; bk, ''blood- 

 hud " ; c, crest of the covering fold ; ec, ectoderm of the embryo ; ./', covering fold 

 (embryo-sac) ; k, paired fold-like rudiment of the gill ; p, upper cavity between the 

 placenta and the embryo; p', actual placental cavity. 



body-musculature and the elaeoblast. As the embryo grows further 

 a marked increase in length takes place, and it thus approaches the 

 adult form, whereas at first its transverse diameter was greater than 

 its longitudinal diameter (cf. Fig. 21 S /> with Fig. 224). This change 

 in shape is specially connected with the change both in shape and 

 relation of the pharyngeal cavity. This rudiment, which may be 

 considered as consisting of two sacs connected by a narrow transverse 



