No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 73 



established in these forms. The two most important factors in 

 establishing the adult relations are the growth of the shell- 

 gland, i.e., dorsal region, and of the foot respectively. The 

 shell-gland assumes the adult relations first owing to its early 

 importance ; the foot or neural axis is established later ; this is 

 in adaptation to its lack of function in the trochophore. 



These axial shiftings have often been referred to. It 

 will, nevertheless, be useful to review shortly the clearest 

 accounts of them. Fol (No. 44) gives a remarkably straight- 

 forward account of the axial shiftings in the pteropods and 

 heteropods. His statement in his pteropod paper is so concise 



hk 



Fig. 12. — Ostrea (after Horst, from Korschelt and Heider). 

 N.A. Neural Axis. D.A. Dorsal Axis. 



that it may be quoted entire (I.e., p. 202). "En effet, aussit6t 

 que les deux feuillets primitifs de I'embryon sont formes, le 

 feuillet externe se met a croitre et a s'etaler beaucoup plus 

 rapidement d'un cote que de I'autre. Ce c6t6 a croissance 

 rapide repond a la region ventrale et posterieure de la larve, 

 a celle ou se trouve, on se le rappelle, la spherule primitive 

 protoplasmique, a celle qui donne naissance au pied et au 

 manteau. Le tissu ectodermique qui occupait dans I'origine 

 le pole formatif subit de la sorte un d6placement relatif et 

 parait remonter le long du dos de I'embryon pour arriver 

 enfin a la region cephalique." 



*' La spherule primitive protoplasmique " to which Fol refers 

 is the posterior macromere. The mantle and the foot, the 

 whole trunk in fact is traced back to the posterior macro- 

 mere. 



