42 VERTEBRATE ZOOLOGY 



the greatly reduced coelom to the atrium. They are segmental organs 

 and occur typically a pair to a metamere in the pharyngeal region. 

 The resemblance between these structures and those of anneUds is 

 fairly close. The muscular system consists of segmental myotomes 

 separated by connective tissue myocommata. The myotomes are 

 chevron-shaped as seen from the side, and resemble those of the 

 fishes. 



The spinal cord has spinal nerves (Fig. 11) that alternate on the 

 two sides and have dorsal and ventral roots. The fin system is very 

 primitive, consisting of a low continuous median fin-fold running un- 

 interruptedly about the tail and ending back of the atriopore. The 

 fin-folds are supported by short connective tissue fin-rays. Paired 

 ridges, called metapleural folds, run along the ventro-lateral portions 

 of the body; they have been thought of as the primordia of paired ap- 

 pendages. The integument consists of a single layer of ectodermal 

 cells and several layers of dermal cells. 



The gonads are simply metameric pouches of the ccelom in the bran- 

 chial region. The eggs and sperm escape by rupture of the body 

 wall into the atrium, and fertilization is external. The sexes are sep- 

 arate. The egg is small and practically yokeless. 



Embryology op Amphioxtjs 



"As an introduction to the study of embryology, and as an 

 indispensable aid to a reasonable appreciation of the value of em- 

 bryological facts, the life-history of Amphioxus provides an ob- 

 ject, which for its capacity of appUcation to ahnost every branch 

 of zoological discussion is perhaps unrivaled. All the funda- 

 mental structures of the body are laid down with schematic 

 clearness." (Willey.) 



The ovum is microscopic and resembles those of many marine in- 

 vertebrates. Spawning occurs at sun-down when simultaneously 

 females and males discharge ova and spermatozoa into the sea-water, 

 where fertihzation occurs. Maturation phenomena resemble those of 

 invertebrates, as do also the cleavage stages, the first two cleavages 

 being from pole to pole (meridional) and the third equatorial, pro- 

 ducing a tetrad of micromeres and a tetrad of macromeres (Fig. 16). 

 The micromere cells divide more rapidly than the macromere cells and 

 a blastula (Fig. 16, F) is formed with smaller ectodermal cells at the 



