444 Miscellaneous. 



opake ; they possess a viscous, temporary outer envelope, and a finer 

 delicate chorion {membrane coquillihe of Vogt). The latter is finely 

 punctured, and appears to be pierced by minute tubes, as in other 

 fishes. The vitellus is furnished with an extremely delicate vitelline 

 membrane. No micropyle could be discovered. 



The segmentation of the vitellus commences six hours after im- 

 pregnation. It implicates the whole of the yelk, and differs in this 

 respect from that which occurs in other fishes, in which only a small 

 portion of the vitellus (formative vitellus) undergoes this transforma- 

 tion. The segmentation of the vitellus in the Lampreys is exactly 

 the same as in the Frogs, and the vitelline membrane furnishes very 

 delicate envelopes for the segments, which are true cells. 



The two first furrows are perpendicular, whilst the third is trans- 

 verse, separating the egg into upper and lower portions. The seg- 

 mentation goes on much more rapidly in the upper half, so that 

 when the segmentation is complete (two days after fecundation), the 

 cells contained in it are much smaller than those in the lower portion. 

 During this period a large cavity is formed in the interior of the egg, 

 but situated almost entirely in the upper part, which it dilates into a 

 thin vesicle, whilst its bottom is formed by the large cells of the 

 lower part of the egg. As in the eggs of the Frogs, this cavity 

 disappears during the further development, and its purpose is not 

 known. 



The first changes which take place after segmentation consist in 

 the increase of the upper part, which grows over the lower and 

 covers it, — not equally on all parts of the circumference, but only on 

 one side, by a margin in the form of a high ridge. At the side of 

 the latter and beneath it, a hollow is formed in the lower part of the 

 egg, corresponding to the anus in the eggs of the Frog. This is the 

 entrance to a second cavity, the primitive alimentary cavity^ which 

 is developed during the diminution of the cavity of segmentation ; it 

 subsequently becomes the anus of the Lamprey, and is therefore the 

 first portion of the fish to make its appearance. No trace of vibratile 

 cilia is distinguishable on the surface of the e^^, which does not 

 exhibit any rotatory movement like that which distinguishes the eggs 

 of the Frog. On the fifth day, dorsal ridges, like those of the Batra- 

 chia, make their appearance, and the dorsal furrow which is situated 

 between them soon closes above ; the extremity of the head now rises 

 distinctly, whilst the anus constantly becomes smaller, but never dis- 

 appears entirely. In the meantime the primitive alimentary cavity 

 extends up to the extremity of the head of the embryo, whilst it dis- 

 appears gradually in the neighbourhood of the anus, from the 

 approximation of the large cells of the lower portion of the yelk, by 

 which it was enclosed. In this way the primitive alimentary cavity, 

 which never exhibits vibratile cilia in its interior, becomes the pha- 

 ryngeal, and subsequently the branchial cavity. At this time also 

 the chorda dorsalis and the heart make their appearance ; the latter 

 only makes sixteen pulsations in a minute. On the sides of the 

 chorda dorsalis appear parts which become the lateral muscles 

 {vertebral divisions of Vogt) ; above it are the commencements of the 



