190 



constricted-off portion of the myocoele. The sperms and 

 ova are discharged into the gonad sac ; and, through pores 

 in its wall, they pass into the atrium, thence out by the 

 atriopore. 



The spawning period is from April to July ; and evening 

 is the spawning time. The eggs are generally fertilised 

 in the sea. 



Development of Amphioxus. 

 Briefly describe the Segmentation of the fertilised Egg. 



The egg contains very little food-yolk; consequently the 

 segmentation or cleavage is nearly equal and complete 

 (holoblastic). The egg is divided vertically into two halves 

 (right and left halves of future embryo). These halves 

 are divided vertically into four blastomeres. The next 

 division, nearly equatorial, results in eight blastomeres ; 

 the four lower ones being larger (macromeres) than the four 

 upper ones (micromeres). Further bisection results in 

 sixteen. Subsequent division is less regular, the segment- 

 ation cavity (blastocoele) increases in size, and a spherical 

 blastula is 'formed. (See description of Blastula, page 14 r 

 Part I.). 



What is the next Stage of Development ? 



The forming of a cup-like two-layered embryo, the 

 gastrula (see Gastrula, page 14, Part I.), by (1) infolding 

 (invagination) of the vegetal hemisphere, and by (2) in- 

 turning (involution) of animal pole cells developed at the 

 dorsal part of the blastopore lip, and accompanied by 

 (3) continuous overgrowth (epiboly) of the lip, whereby 

 the gastrula is elongated and its opening reduced. The 

 embryo is now elongate, flattened dorsally and convex 

 ventrally, with a small posterior opening (the blastopore) 

 dorsalwards. 



Explain the further Development of the Embryo to the time 

 when it is hatched. 



Along the dorsal line, the ectoderm (epiblast) thickens 

 to form the medullary plate, which sinks in and is detached ; 

 and the lateral ectoderm grows over it. The plate widens, 

 becomes grooved ; and ultimately, by the union of its 

 sides (medullary folds), it is formed into a tube, the central 

 (medullary) canal of the nervous system. The overgrowth 

 of ectoderm extends backwards from the hind end ; and 

 so the blastopore becomes the short neurenteric canal or 

 communication betweeiT the archenteron and the medullary 

 canal, which ends anteriorly in the neuropore. The neuro- 



