66 



MARINE BIOLOGICAL LABORATORY. 



two halves, which are at first completely separate, i.e.y 

 by concrescence. And each band grows at its posterior 



)atii- 



i&. 





Fig. 3. 



tip, the somites being progressively developed from this 

 point forwards, i.e., by apical growth. 



I think it must be clear, therefore, that the investiga- 

 tion, from an embryological point of view, of metamer- 

 ism, of apical growth, and of concrescence is insepara- 

 ble from the study of the whole series of phenomena 

 relating to the mesoblast formation ; and that a close 

 study of the origin of the mesoblast in annelids is of 

 great importance to comparative embryology. If we 

 turn now to the literature of the subject, we find the 

 utmost confusion, the most extraordinary differences of 

 opinion among the various authors, respecting almost 

 every detail of the subject. We find the mesoblast de- 

 scribed by one author as arising wholly from the ecto- 

 blast, by another as arising from the entoblast, by 

 others as arising from both layers, or again as arising 



