4:0 PROFESSOR A. M. MARSHALL. 



attracted much attention, since in the absence, probably inevitable, of 

 satisfactory paleeontological evidence, they afford us the sole available 

 clue to the determination of the mutual relations of the large groups 

 of animals, or of the points at which these diverged from one another. 



In attempting to interpret these early ontogenetic stages as actual 

 ancestral forms, beyond which development at one time did not 

 proceed, we must keep clearly in view the various disturbing causes 

 which tend to falsify the ancestral record ; such as the influence of 

 food yolk, or of habitat, and the tendency of diminution in size to give 

 rise to simplification of structure, a point of importance if it be granted 

 that these free larvae are of smaller size than the ancestral forms to 

 which they correspond. 



If, on the other hand, in spite of these powerful modifying causes, 

 we do find a particular larval form occurring widely and in groups 

 not very closely akin, then we certainly are justified in attaching 

 great importance to it, and in regarding it as having strong claims to 

 be accepted as ancestral for these groups. 



Concerning these larval forms, and their possible ancestral 

 significance, our knowledge has made no great advance since the 

 publication of Balfour's memorable chapter on this subject ; and I 

 propose merely to allude briefly to a few of the more striking- 

 instances. 



The earliest, the most widely spread, and the most famous of larval 

 forms is the gastrula, which occurs in a simple or in a modified form 

 in some members of each of the large animal groups. It is generally 

 admitted that its significance is the same in all cases, and the evidence 

 is very strong in favour of regarding it as a stage ancestral for all 

 Metazoa. The difficulty arising from its varying mode of develop- 

 ment in different forms is, however, still unsolved and embryologists 

 are not yet agreed whether the invaginate or delaminate form is the 

 more primitive. In favour of the former is its much wider occurrence ; 

 in favour of the latter the fact that it is easy to picture a series of 

 stages leading gradually from a unicellular protozoon to a blastula, a 

 diblastula, and ultimately a gastrula, each stage being a distinct 

 advance, both morphological and physiological, on the preceding 

 stage ; while in the case of the invaginate gastrula it is not easy to 

 imagine any advantage resulting from a flattening or slight pitting in 



