SALPIDAE FORMS WITHOUT COVERING FOLDS. 429 



the brood-sac which can still be recognised for some time a's a kind 

 of corpus luteum. The embryo, at birth, passes out through the 

 atrial aperture of the parent. 



The development of the final shape of the body goes hand in hand 

 with the development of the placenta. The embryo at first formed 

 a cone projecting into the atrial cavity of the mother (Fig. 212), its 

 principal axis corresponding to the future dorso-veiitral axis. It now 

 lengthens at right angles to this axis, i.e., in accordance with its 

 future longitudinal axis (Fig. 214). It soon becomes cylindrical and, 

 after the cellulate mantle has developed, resembles a tetragonal prism. 

 The mantle-substance develops in just the same way as in the 

 Ascidians (p. 355). It arises on the outer surface of the ectoderm 

 as a secretion into which single cells soon wander. Finally, the two 

 conical processes characteristic of Salpa democratica (solitary form) 

 develop (Fig. 262, p. 495). 



The nervous system has been seen to arise as a solid ingrowth of 

 the ectoderm.* This soon severs its connection with the latter, a 

 cavity develops within it, and it becomes a vesicle characterised by 

 its size and the thickness of its walls (Fig. 214, n). KOWALEVSKY 

 (No. 96) was aware of the fact that this vesicle lengthens later and 

 is indistinctly divided by constrictions into three consecutive parts 

 which show a certain resemblance to the primary cerebral vesicles 

 of the vertebrate embryo (Fig. 214 B, n). The anterior vesicle 

 becomes closely connected anteriorly with the adjacent wall of the 

 pharynx, and this connection which is at first solid soon develops 

 a lumen which puts the neural cavity into communication with the 

 pharyngeal cavity. The canal thus formed is the first rudiment of 

 the future ciliated pit (Fig. 216, fi). While this organ develops 

 further, the walls of the ganglionic vesicle thicken, it shortens, its 

 lumen disappears and the vesicular rudiment thus gradually assumes 

 the character of the definitive ganglion of the adult. A conical 

 process rising on the dorsal side of the ganglion on which three 

 accumulations of pigment appear represents the rudiment of the 

 eye. 



Further details as to the development of the eye have recently been pub- 

 lished by METCALF (No. 99) and BUTSCHLI (No. 94). t The eyes seem to develop 

 differently, not only in the different species but also in the solitary and the 

 colonial forms of the same species. According to BUTSCHLI, the simplest form 



*[See footnote, p. 425. ED.] 



I [See also METCALF in Brooks' Monograph (No. I.), and GOPPEKT 

 No. 94a). ED.] 



