THE BIOGENETIC LAW 



171 



the pliyletic stage represented by the Sehizopods appears as an 

 ontogenetic stage, just before the final metamorphosis of the larva 

 to the perfect animal. This is the ease in most of the marine 

 Decapods, in those forms wliich do not go through the whole course 

 of their development within the egg, but emerge as Zosea larvae, 

 or e\-en, as in Peneua potimirim, as nauplii. In the last-named 

 species (Fig. 109) the ontogeny contains at least three stages which 

 must have lived, perhaps not as mature forms, but as primitive larval 

 forms, for unthinkable ages — the stage of the nauplus (Fig. 109 A), 

 that of the Zorea (Fig. 109 B and C), and that of the Schizopod 

 (Fig. 109, D) ; from this la.st the fully 

 developed Decapod Crustacean arises 

 (Fig. 109, E). 



We are, therefore, justified in 

 saying that here the racial evolution 

 is recapitulated in the individual de- 

 velopment, although condensed and 

 shortened in pi-oportion as more numer- 

 ous stages of the pliyletic development 

 are gone through within the egg, for 

 there the different stages can succeed 

 each other more rapidly and directly 

 than in a metamorphosis of the free- 

 swimming larvw, since these must 10,,,. i,^. Zo«.a-larva of a Grab, 

 procure their own material for their f^ftei- R. Hertwig. i-v, the already 



j; n .1 . functional anterior appendages — 



further growth and their metamor- antenn£e,m.<indibles,andswimming- 



phosis, while the yolk of the egg '«s^; .^^--s-J^-T, rudiments ot-^the 



^ / _ *' _ _ ='' posterior appendages 01 the cepnalo- 



supplies a store of material which is thorax {Cpk). Ahd, the abdomen. 



sufficient for the production of a whole ^; ^^Crt."^ ^^' "'™^''''' ^''' '^'' 

 series of successive stages. 



For this reason it inevitably resulted that the sharply defined 

 characters of the phyletic stages were more and more lost as 

 soon as they were transferred from larval stages to stages in 

 embryogenesis. For, in the first place, these sharply defined charac- 

 ters, such as the spines of the Zosea larva, or the swimming bristles of 

 the ' oars,' or the shape of thorax or abdomen characteristic of certain 

 species, are adapted to a free life, and would be valueless in an 

 embryonic stage ; and secondly, in the transference of the free larval 

 stages to embryonic development the greatest possible condensation 

 and abbreviation of the stages must have been striven for, which 

 could only come about by a continual mutual adaptation of the 

 embryonic parts to one another, involving the suppression of every- 



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