40 



EMBRYOLOGY 



egg not a polyp, but a free-swimming larva, which passes 

 into the form of a mednsa by a simple metamorphosis Qiypo- 

 gcnetic forms with suppressed alternation of generations). 



Metagenetic Medusae. — We begin with the description 

 of the better-known cases of development of the eggs of 

 hydroid medusce, and follow principally the accounts of 

 Claus (No. 3) and Metschnikoff (No. 12). The spheroidal 

 eggs of the craspedote medusae are for the most part colour- 

 less, transparent, and destitute of a membrane. There may 

 be distinguished in them a layer of ectoplasm, consisting of 

 a viscid formative yolk, and an endoplasm filled with coarse 



Fig. 13.— Development of the e^g of Rathkea fasciculata ("after Metschnikoff). 

 A, an egg immediately after deposition ; r, polar corpuscles ; B, stage of first divi- 

 sion ; C, eight-cell stage ; D, blastula-stage in optical section ; E, planula-stage vrith 

 formation of entoderm, en. 



i 



granules of nutritive yolk (Fig. 13 A). After fertilizatio 

 they undergo a total, and in most cases a nearly equal, cleavage. 

 By the formation of the first two meridional and mutually 

 perpendicular furrows (extending from the animal to the 

 vegetative pole, Fig. 13 JB), there arise four blastomeres 

 placed in the form of a cross, and by a succeeding equatorial 

 furrow there is produced an eight-cell stage (Fig. 13 0), 

 while two additional meridional furrows lead to the formation 

 of a sixteen-cell stage. Only in certain cases (^quorea) is 

 the cleavage more unequal, the blastomei^s of the animal 



