14 F.YE. 



a manner not clearly made out) round the right side of the vesicle 

 till it comes to the crista ; to which it is at first attached by a 

 narrow pedicle. The fully developed eye (figs. 8 vr. and 9, 0) consists 

 of a cup-shaped retina, which forms a prominence slightly on the 

 right side of the posterior part of the dorsal wall of the anterior 

 cerebral vesicle, and of refractive media. The retina is formed of 

 columnar cells, the inner ends of which are imbedded in pigment. 

 The refractive media of the eye are directed towards the cavity of 

 the cerebral vesicle, and consist of a biconvex lens and a meniscus. 

 Half the lens is imbedded in the cavity of the retina and surrounded 

 by the pigment, and the other half is turned toward a concavo-convex 

 meniscus which corresponds in position with the cornea. The de- 

 velopment of the meniscus and lens is unknown, but the retina 

 is formed (fig. 8 V. ) as an outgrowth of the wall of the brain. At 

 the inner ends of the cells of this outgrowth a deposit of pigment 

 appears. 



The trunk section of the spinal cord (fig. 9, JV) is separated by a 

 sharp constriction from the brain. It is formed of a superficial layer 

 of longitudinal nervous fibres, and a central core of ganglion cells. 

 The layer of fibres diminishes in thickness towards the tail, and 

 finally ceases to be visible. Kupffer detected three pairs of nerves 

 passing off from the spinal cord to 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 under- 

 goes a further development which finds its only complete parallel amongst 

 Chorclata 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 con- 

 tinuous axis of hyaline substance. The remnants of the cells with their 

 nuclei form a sheath round the hyaline axis (fig. 8 VI. ch.}. Whether the 

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



