EMBRYONIC DEVELOPMENT. 105 



Ascidians and of Amphioxus marks a distinct epoch in 

 the progress of the science of embryology. 



EMBRYONIC DEVELOPMENT. 

 Fertilisation and Segmentation of the Ovum. 



The breeding-season of Amphioxus extends, in the Med- 

 iterranean, from spring to autumn. 



The gonadic pouches become very much distended by 

 the ripening of the ova and spermatozoa in the respective 

 sexes,, and finally burst, discharging their contents into the 

 atrial cavity, whence they 

 reach the exterior through 

 the atriopore. 1 At the time 

 of complete sexual matu- 

 rity the gonads become so 

 large that the atrium is 

 used up to its utmost 

 capacity, and its walls be- 

 come stretched to such an 

 extent that the meta- 



pleural folds are flattened Fig. 52. -Unfertilised ovum 



oxus. Magnified about 750 diameters. 

 Up against the sides of the (After LANGERHANS.) 



i i a. IUIK gianuie:=. /. romcie. 71. i\u- 



y* cleus (germinal vesicle), with nucleolus. 



The OVUm is Semi- $ P roto P' asm i c area, free from yolk gran- 

 ules, surrounding the nucleus. 



opaque, contains granules 



of yolk equally distributed throughout its substance, and 

 is surrounded by a cellular membrane known as \.}\Q follicle 

 of the egg, and sometimes less accurately spoken of as 

 the mtclline membrane (Fig. 52). 



Spawning, when it occurs, invariably takes place at sun- 

 down, i.e. between five and seven o'clock in the evening, 

 and never, so far as is known, at any other time. 



