174 DEVELOPMENT OF AMPHIOXUS. 



that they are potentially paired, the first of them in all 

 probability being actually paired (with the club-shaped 

 gland). 



In the second place, after the closure of a number of 

 the primary slits, the so-called critical stage occurs with 

 eight pairs of gill-slits. This is another resting phase in 

 the development, and marks the turning-point from the 

 larval to the adolescent period. Subsequently the addi- 

 tion of new gill-slits behind those already present com- 

 mences and goes on indefinitely throughout life. 



Counting in the first pair of slits (i.e. first primary slit 

 plus club-shaped gland) which is destined to atrophy, we 

 must regard it as probable that the proximate common 

 ancestor of Amphioxus and the higher Vertebrates was 

 characterised by the presence of from nine to fourteen 

 pairs of gill-clefts, although it is also probable that there 

 was a variable tendency to add to this number by fresh 

 perforations. 



NOTES. 



1. (p. 105.) It is unaccountable how there can have been 

 conflicting statements as to the ejection of the genital products 

 (male and female) through the atriopore. It was first observed by 

 DE QUATREFAGES in 1845, an( ^ ms observations have since been 

 fully confirmed by PAUL BERT, A. WILLEY, and E. B. WILSON. On 

 the other hand, both KOWALEVSKY and HATSCHEK affirm that they 

 are discharged through the mouth. It is to be regretted that two 

 such eminent observers should have committed this error, since it 

 is difficult to eradicate it from the text-books. 



2. (p. 115.) The primitive endoderm cells in the neighbour- 

 hood of the neurenteric canal apparently retain an undifferentiated 

 character, until the completion of the myotome-formation. In the 

 young embryo they are to be observed in transverse section in pro- 

 cess of division, numbers of karyokinetic figures being present. 

 But the cells divide without regard to the median plane of sym- 



