18 PROFESSOR A. M. MARSHALL. 



effects of loss of food yolk. It has been suggested that the extraordinary 

 nature of the segmentation of the egg of Peripatus capensis, made 

 known to us through Mr. Sedgwick's admirable researches, may be due 

 to loss of food yolk ; a suggestion which receives support from the 

 long duration of uterine development in this case. 



Our knowledge is very imperfect as to the ease with which food 

 yolk may be acquired or lost ; but until our information is more 

 precise on this point, it seems unwise to lay much stress on suggested 

 pedigrees which involve great and frequent alternations in the amount 

 of food yolk present. 



Of causes other than food yolk, or only indirectly connected with it, 

 which tend to falsify the ancestral history, many are now known, but 

 time will only permit me to notice the more important. These are 

 distortion, whether in time or space ; sudden or violent metamorphosis ; 

 a series of modifications, due chiefly to mechanical causes, and which 

 may be spoken of as developmental conveniences ; the important 

 question of variability in development ; and finally the great problem 

 of degeneration. 



Concerning distortions in time, all embryologists have noticed the 

 tendency to anticipation or precocious development of characters which 

 really belong to a later stage in the pedigree. The early attainment of 

 the cyclical form in the shell of Orhitolites complanata is a case in point ; 

 and Wiirtenberger has specially noticed this tendency in Ammonites. 

 Many early larvse show it markedly, the explanation in this case being 

 that it is essential for them to hatch in a condition capable of 

 independent existence, i.e., capable, at any rate, of obtaining and 

 digesting their own food. 



Anachronisms, or actual reversal of the historical order of develop- 

 ment of organs or parts, occur frequently. Thus the joint surfaces of 

 bones acquire their characteristic curvatures before movement of one 

 part on another is effected, and before even the joint cavities are 

 formed. 



Another good example is afforded by the development of the 

 mesenterial filaments in Alcyonarians. Wilson has shown in the case 

 Renilla that in the development of an embryo from the egg the 

 six endodermal filaments appears first, and the two long ectodermal 

 filaments at a later period ; but that in the formation of a bud this 



