THE DEVELOPMENT OF LEPIDOSIREN PAEADOXA. 27 



the continued increase in area of the small-celled outer layer 

 of the egg due to the backgrowth of the upper lip, and 

 under this invagination goes on in the ordinary way. This 

 is, it appears to me, the real significance of the backgrowth. 

 It is a phenomenon directly associated with the increase in 

 bulk of the macromeres. If this were true, we should find it 

 become more and more pronounced as a developmental 

 feature with increase in the quantity of yolk. This is, I 

 think, what we do find, and we can also understand on this 

 view why recent observers have failed to find such a process 

 taking place in Amphioxus. 



I do not propose to enter at length into the controversy 

 which has raged over the parts played by invagination, split- 

 ting, downgrowth of dorsal lip, etc., in the gastrulation of 

 vertebrates. Much of the evidence that has been brought 

 seems to me unreliable, resting as it does on such characters 

 as size of cells, size of yolk-granules, presence of pigment — 

 characters which appear to me to be in great part merel}' the 

 expression of greater or less metabolic activity for the time 

 being, and which cannot therefore safely be used as criteria 

 in treating of morphological questions. 



Apart from these, the evidence afforded by the study of 

 sections is of such a character that its interpretation is liable 

 to be seriously affected by the observer's preconceived ideas. 

 As regards observations on the living egg, many of the 

 methods also seem opeu to tLe influence of very serious 

 disturbing factors, either of a traumatic nature or of a 

 simple physical character, such as movement of the egg as 

 a whole, brought about by shifting of the centre of gravity 

 due to the change in the relative extent and position of 

 archenteric and segmentation cavities. The only really 

 reliable method of investigation appears to be that of 

 Kopsch,^ where the developing egg is submitted to pro- 

 longed photographic exposures, and the surface-cell move- 

 ments worked out on the pictures so obtained. 



» 'Veih. Auat. Ges.,' 1S95, p. ISl ; and ' S. B. Ges. naturf. Tieuude 

 Berlin,' 1895, p. 21. 



