EMBRYOLOGICAL INVESTIGATIONS 269 



provisional, until we gain more complete and exact 

 knowledge concerning the conditions under which 

 the power of independent development can be 

 exerted, not only in Amphioxus, but in other 

 animals as well. 



The present paper is merely a record of some 

 recent investigations, and has no pretence to 

 completeness. These researches however all 

 tend in one direction, namely to emphasise the 

 fact that the development of animals takes place 

 in far more varied manner than is generally 

 supposed, and that marked differences may occur 

 both in the earlier and the later stages of the 

 embryological history, not merely in allied genera 

 and species, but even between individual members 

 of the same brood. I will conclude with a brief 

 summary of the principal modes in which the 

 development of a Metazoon may be effected. It 

 should be noted that the adult structure as a rule 

 gives no indication of the particular mode in which 

 development has occurred : a sponge developed 

 from a gemmule is, so far as we know, indistin- 

 guishable in its adult form from one developed from 

 a fertilised egg. 



AN ADULT METAZOON MAY BE DEVELOPED. 



i. From a group of (mesoblast ?) cells, 

 which are originally independent of one 

 another, and of which none are fertilised. 

 Examples: The gemmules of Sponges, 

 or the statoblasts of Polyzoa. 



