444 TUNICATA. 



rudiment, is known as the pericardia! rudiment (pc) in the strict 

 sense of the term, while the posterior strand has been called by 

 SALENSKY the sub-pericardial strand (Fig. 224 A, x, sp). In the 

 further course of development, the posterior strand breaks up into 

 its elements, which are probably transformed into blood-corpuscles. 

 The pericardial rudiment proper, which was originally a solid strand, 

 becomes a tube known as the pericardial sac, the superficial cells 

 uniting to form an epithelial wall, while the inner cells lose their 

 cohesion and, as it appears, disintegrate. The heart (Fig. 222, h) 

 develops in a way similar to that described for the other Tunicates, 

 as an invagination of the wall of the pericardial sac (pc). The lumen 

 of this invagination then becomes the cavity of the heart. 



^r 



JIG. 224. Two older embryos of Salpa pinnata (after SALENSKY). cl, cloaca ; c/, 

 enteric rudiment ; e, atrial aperture ; eb, elaeoblast ; es, endostyle ; /, covering fold ; 

 Ji, ciliated pit ; h, heart ; i, branchial aperture ; k, gill ; m, muscle-hoops ; >/. 

 nervous system ; p, pericardial sac ; ph, pharynx ; pi, placenta ; s, proliferating 

 stolon ; sp, sub-pericardial cell-strand. 



The muscle-hoops arise from the mesenchyme-cells present in the 

 primary body-cavity. Some of these become arranged on the outer 

 .surface of the wall of the respiratory cavity to form a layer (muscle- 

 plate) in which, at an early stage, thicker and thinner transverse 

 -zones can be distinguished. The fenestration observed by LEUCKART 

 (p. 431), by means of which the hoops are separated from one another 

 {Fig. 224, m) takes place in the thinner zones. The muscle-layer is 

 said first to develop on the dorsal side of the body and to proceed 

 thence towards the ventral side. It should be mentioned that a 



