EMBRYOLOGY 379 



Compared with such an animal as Scyllium, for example, it is very 

 simple in structure. It has no cranium nor sense organs, nor is its 

 brain nearly so well developed. No cartilaginous skeleton is present, 

 and even though it has a mouth, this is a circular orifice not provided 

 with any structures that can be looked upon as jaws. No sign of 

 paired fins is present, and all its systems are much less complicated 

 with the one exception of the gill clefts, which are very numerous. 

 The actual structure of the adult, however, does not concern us here, 

 and it is sufficient to note that while it is undoubtedly highly 

 specialised along certain lines, it is a representative of one of the most 

 primitive groups of Chordates alive to-day, and so of considerable 

 interest in comparative anatomy. Its embryological changes are 

 also specialised, but, nevertheless, they illustrate certain funda- 



jnf 



FIG. 127. Amphioxus, general view. From Bourne. 



A, an., anus ; aip., atriopore ; c., caudal fin ; ci, buccal cirrhi ; df, dorsal fin ; e., eyespot ; 

 fr, fin-rays ; g'., g 26 ., the twenty-six pairs of gonadial pouches ; m'., the first, m 36 ., the thirty- 

 sixth, w 52 ., the fifty-second myotomes ; n., neural tube ; nch., notochord ; vel., velum, in front of 

 it are the finger-like processes of the wheel organ ; ves., vestibule ; ./., ventral fin. 



mental points in a very clear manner, and it is for that reason we 

 treat of it here. 



The fertilised ovum of Amphioxus is a minute spherical 

 body about *io mm. in diameter. While the yolk that it contains 

 is more abundant towards one pole, and so, strictly speaking, it is 

 telolecithal ; yet there is so little present, and it is not entirely 

 confined to one end, that it is often considered as homolecithal. 

 Shortly after fertilisation the second polar body is extruded at what 

 is termed the animal' pole of the egg, and this is followed later by 

 the first cleavage. The first division is holoblastic, that is to say, it 

 passes completely through the ovum, and it takes place in a plane 

 passing through the middle of the two poles. Shortly after, the 

 second cleavage takes place, at right angles to the first, but still 

 through the middle of the poles. Thus, after these two meridional 

 divisions, the egg comes to consist of four equi-sized cells, each being 

 half in the animal and half in the vegetative pole. The third 

 division takes place at right angles to the other two, and is often 

 termed equatorial, although, as a matter of fact, it is nearer one pole 



