ILLUSTRATIONS 



1. Eight views of the maturation of the male cells of Lepidosircn paradoxa . 5 



2. Nine stages in the transformation of a spermatic! into a spermatozoon . 8 



3. Four stages in the maturation and fertilization of the egg of Orepidula 



plana ......... 10 



4. Two stages in the first division of the fertilized egg of Crepidula plaim . 11 



5. Polar view of the first maturation division of the male germ cells of Alydus 



pilosulus in order to show tetrads ...... 12 



Second maturation division of the male germ cells of Protenor belfrayci . 14 

 Second maturation division of the male germ cells of Euschistus variolaris . 14 

 The larva and adult female of Porlunion maenadis . . . .23 



The stalked larva and adult form of Antedon multispina . . .24 



The larva and just-metamorphosed form of the Plaice (Pleuronectes platcssa] 25 

 Unripe egg of Limulus polyphemus. An example of a centrolecithal egg . 27 

 The ripe egg of Stront/ylocentrotus lividus. An example of an alecithal egg 27 

 Two stages in the segmentation of the egg of Sycandra raphanus . . 38 



View of the embryo of Grantia labyrinthica in the blastula stage lying in 



the embryonic chamber of the mother . . . . .39 



View of the embryo of Grantia labyrinthica in a later stage of development 



than that represented in Fig. 14 . . . . . .39 



Section of a portion of Grantia labyrinthica . . . . .40 



The Amphiblastula larva of Grantia labyrinthica ' . . . .40 



Two stages in the fixation of the larva of Sycandra raphanus . . 41 



An early stage in the metamorphosis of the Ascon stage of Syrt/m/ri/ 



raphanus into the adult ....... 41 



A late stage in the metamorphosis of the Ascon stage of Sycandra wtyhanus 



into the adult condition ....... 42 



Longitudinal sections through the free-swimming larva ofOsat-i-fffa luln/lnrix 



in two stages of its development and its fixation . . . .44 



Seven stages in the metamorphosis and fixation of the larva and growth of 



the young sponge of Plakina monolopha . . . . .45 



Two sections of the body-wall of the larva of Plakina monolopha, to show 



the distinction between archaeocytes and mesenchyme . . .46 



Longitudinal section through the Amphiblastula larva of Esperia lorcnzi . 46 

 Longitudinal sections of the Amphiblastula larva, the just-fixed larva, and 



the young sponge of Leucosolenia variabilis . . . .48 



Section through a gemmule-beariug individual of Ephydatia blembinyia . 49 

 Three stages in the formation of the gemmules of Ephydatia blembinyia . 50 

 Two methods of formation of the blastula in Tulularia mescmbryanthcmum 55 

 Formation of endoderm in Tubidaria mescmbryanthcmu/n . . 55 



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