UROCHORDA. 17 



the muscles of the tail. The foremost of these arises at the 

 boundary between the trunk and the tail, and the two others at 

 regular intervals behind this point. 



The mesoblast and muscular system. It has already been 

 stated that the lateral walls of the archenteron in the tail give 

 rise to muscular cells. These cells lie about three abreast, and 

 appear not to increase in number ; so that with the growth 

 of the tail they grow enormously in length, and eventually 

 become imperfectly striated. The mesoblast cells at the hinder 

 end of the trunk, close to its junction with the tail, do not 

 become converted into muscle cells, but give rise to blood 

 corpuscles ; and the axial remnant of the archenteron undergoes 

 a similar fate. According to Kowalevsky the heart is formed 

 during larval life as an elongated closed sack on the right side of 

 the endostyle. 



The notochord. The notochord was left as a rod formed of 

 a single row of cells, or in As. canina and some other forms of 

 two rows, extending from just within the border of the trunk to 

 the end of the tail. 



According to Kowalevsky, Kupffer, Giard, etc. the notochord undergoes 

 a further development which finds its only complete parallel amongst 

 Chordata in the doubtful case of Amphioxus. 



There appear between the cells peculiar, highly refractive discs (fig. 8 v. 

 Chs). These become larger and larger, and finally, after pushing the 

 remnants of the cells with their nuclei to the sides, coalesce together to form 

 a continuous axis of hyaline substance. The remnants of the cells with 

 their nuclei form a sheath round the hyaline axis (fig. 8 VI. c/i.). Whether 

 the axis is to be regarded as formed of an intercellular substance, or of a 

 differentiation of parts of the cells is still doubtful. Kupffer inclines to the 

 latter view : the analogy of the notochord of higher types appears to me to 

 tell in favour of the former one. 



The alimentary tract. The anterior part of the primitive 

 archenteron alone retains a lumen, and from this part the whole 

 of the permanent alimentary tract (mesenteron) becomes deve- 

 loped. The anterior part of it grows upwards, and before 

 hatching an involution of the epiblast on the dorsal side, just in 

 front of the anterior extremity of the nervous system, meets and 

 opens into this upgrowth, and gives rise to the permanent mouth 

 (fig. 8 v. o). 



B. III. 2 



