DEVELOPMENT OF PETBOMYZON TLUVIATILIS. 187 



The second variety of muscle-fibre met with in the Lamprey- 

 seems to be exclusively derived from the ventral unsegmeated 

 mesoblastic plate^ and from the walls of the head cavities. 

 The muscles with this origin are those which serve to move the 

 lips, the velum and the other structures of the mouth, and 

 certain muscles connected with the gill apparatus, and prob- 

 ably the muscles of the eye. These latter have the same 

 histological structure, but owing to the fact that the eye does 

 not develope until the Lamprey stage, no eye muscles appear 

 till very late in the life of the Ammocoete and I have conse- 

 quently been unable to follow their development. 



The muscle-fibres of this second variety of muscle tissue, 

 consist of long tubular cells, cylindrical in shape, with a 

 medulla of clear substance which does not stain, and a cortex 

 which is thickened at intervals by longitudinal rods. These 

 give the cortex a beaded appearance in transverse section. 

 The medulla contains the nucleus, which stains deeply. This 

 is at first single, but subsequently divides until a row of nuclei 

 occupy the axis of the muscle-fibre, in some cases so closely 

 packed as almost to touch. It will be noticed that these 

 muscle-fibres resemble in the minute structure the first stage 

 in the development of the muscles forming the myomeres. 

 These muscle-fibres are transverse striated. 



The fibres of the heart belong to this second variety, and are 

 developed from the same part of the mesoblast. They, however, 

 possess certain peculiarities which will be described after the 

 formation of the heart has been considered. 



The Heart. 

 The first appearance of the body cavity as a space takes 

 place in the region behind the posterior gill-cleft and in front 

 of the liver. The part of the embryo lying in front of this 

 region is at an early stage raised from the posterior half by the 

 backward growth of the head fold, and the embryo lies within 

 the egg-shell bent in half, the angle of the bend being just in 

 that region where the heart is subsequently formed. By this 

 means all those parts in front of the liver are free from the 

 yolk-bearing cells, and the lining cells of the mesenteron all 



