EMBR YOXIC BEVEL OPMEXT. 



105 



Ascidians and of Amphioxus marks a distinct epoch in 

 the progress of the science of embryology. 



EMBRYONIC DEVELOrMEXT. 



Fertilisation and Scg'>nc!itation of the Ovum. 



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

 iterranean, from spring to autumn. 



The gonadic pouches become very much distended bv 

 the ripening of the ova and spermatozoa in the respective 

 sexes, and finall}- burst, discharging their contents into the 

 atrial cavit}-, whence they 



reach the exterior through 

 the atriopore.^ At the time 

 of complete sexual matu- 

 rity the gonads become so 

 large that the atrium is 

 used up to its utmost 

 capacit}-, and its walls be- 

 come stretched to such an 

 extent that the meta- 

 pleural folds are flattened 

 up against the sides of the 

 body. 



The ovum is semi- 

 opaque, contains granules 

 of yolk equally distributed throughout its substance, and 

 is surrounded b^■ a cellular membrane known as thcfolliclo 

 of the egg, and sometimes less accurately spoken of as 

 the vittlliut nionbraiio (Fig. 52). 



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

 down, — i.L\ between five and seven o'clock in the evening, 

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



Fig. 52. — UnfertiiisoJ ovum oi Amphi- 

 oxus. M.ignified about 750 di.imeTers. 

 (Arter L.\NGERH.\NS.1 



j'. Yolk grLinules. f. Follicle. /;. Nu- 

 cleus (germinal vesicle), with nucleolus. 

 /. Protoplasmic area, free from }-olk gran- 

 ules, surrounding the nucleus. 



