M. A. Miiller on the Development of the Lampreys. 299 



The hinder portion of the body is thick, and contains the 

 vesicular intestine, which is still filled with cells of segmenta- 

 tion. A yelk-sac is never present. The long neck begins 

 to move, and at its base the heart is seen, without a pulsatine: 

 bulb. 



About the eighteenth day the young animal escapes from the 

 egg, when it is white and opake ; but its substance gradually 

 becomes clear, until the movement of the blood can be recog- 

 nized, when it begins to develope pigment. The brain and 

 spinal cord resemble a constricted thread, thickened anteriorly. 

 The eyes appear as dark points on the sides of the brain. The 

 neck exhibits eight clefts, of which the anterior soon closes, andthe 

 cavity of the mouth is united with the branchial cavity by a small 

 opening. The intestine consists of a very fine membrane, covered 

 with a very long bacillar epithelium. Along the back of the 

 intestine runs a fold which receives a vessel, and the ureters rise 

 on the dorsal side of the intestine, and form but few ramifica- 

 tions, in which ciliary movement is seen. The dorsal wall of 

 the mouth has at first two, but afterwards several papillar 

 elevations. In front of the heart is a longish oval organ, like 

 a vesicle, and divided down the middle ; this becomes the mus- 

 cular portion of the sucking apparatus of the Lamprey. 



A muscular veil is now developed in the mouth, which pre- 

 vents the exit of water; and the papillse of the upper surface of 

 the mouth increase in number and form branches, constituting a 

 sort of net which prevents the entrance of foreign bodies. At 

 this period the author was surprised by the great similarity of 

 his young fishes with those of the genus Ammocwtes, which 

 occur in the same waters with the Lampreys, and for some time 

 he endeavoured in vain to find any difi'erence between them. 

 After keeping them for two years they died, without exhibiting 

 any tendency to take on the form of their parents ; and during 

 the whole of this period they appeared to be genuine Arnmocoetes. 

 The author was therefore led to imagine, that the supposed 

 genus Ammocostes was in reality founded upon the young of 

 Petrojnyzon. 



To ascertain the correctness of this supposition he sought for 

 Ammocoetes in course of metamorphosis, and found them in a 

 condition which distinctly showed their intermediate state. The 

 silvery lustre of the skin which distinguishes the Lampreys was 

 already perceptible, and the dorsal fin was elongated. The eye 

 was distinct, but in some individuals was still dull, whilst in 

 others it was perfectly clear. The mouth was narrower than in 

 the true Ammoca>tes ; in the latter it measures SJ millimeters, 

 during the metamorphosis 3, and in the fully developed animal 

 in spring 5i. The distance of the nasal opening from the 



