4 EMBRYOLOGY. 



relative value to be attached to the results of isolated observations, 

 and generally to construct a science out of the rough mass of 

 collected facts. It moreover enables each observer to know to what 

 points it is important to direct his attention, and so prevents that 

 simple accumulation of disconnected facts which is too apt to clog and 

 hinder the advance of the science it is intended to promote. 



In the department of Phylogeny the following are the more im- 

 portant points aimed at. 



(1) To test how far Comparative Embryology brings to light 

 ancestral forms common to the whole of the Metazoa. Examples 

 of such forms have been identified by various embryologists in the 

 ovum itself, supposed to represent the unicellular ancestral form 

 of the Metazoa : in the ovum at the close of segmentation regarded 

 as the polycellular Protozoon parent form : in the two-layered 

 gastrula, etc., regarded by Haeckel as the ancestral form of all the 

 Metazoa 1 . 



(2) How far some special embryonic larval form is constantly 

 reproduced in the ontogeny of the members of one or more groups of 

 the animal kingdom ; and how far such larval forms may be 

 interpreted as the ancestral type for those groups. 



As examples of such forms may be cited the six-limbed Nauplius 

 supposed by Fritz Miiller to be the ancestral form of the Crustacea ; 

 the trochosphere larva of Lankester, which he considers to be common 

 to the Mollusca, Vermes, and Echinodermata : the planula of the 

 Ccelenterata, etc. 



(3) How far such forms agree Avith living or fossil forms in the 

 adult state; such an agreement being held to imply that the living or 

 fossil form in question is closely related to the parent stock of 

 the group in which the larval form occurs. It is not easy to 

 cite examples of a very close agreement of this kind between the 

 larval forms of one group and the existing or fossil forms of another. 

 The larvae of some of the Chaetopoda with long provisional seise resemble 

 fossil Chsetopods. The Rotifers have many points of resemblance to 

 the trochosphere, especially to that form of trochosphere characteristic 

 of the Mollusca. The Turbellarians have some features in common with 

 the Ccelenterate planula. Some of the Gephyrea in the presence of a 

 prasoral lobe resemble certain trochosphere types. The larva of the 

 Tunicata has the characters of a simple type of the Chordata. 



Within the limits of a single group agreements of this kind are 

 fairly numerous. ]n the Craniata the tadpole of the Anura has its 

 living representative in the Pisces and perhaps especially in the Myxi- 

 noids. The larval forms of the Insecta approach Peripatus. The 

 stalked larva of Comatula is reproduced by the living Pentacriuus 

 and Rhizocrinus etc. Numerous examples of the same phenomenon 

 are found amongst the Crustacea. 



1 The value of these identifications as well as of those helow is discussed in its 

 appropriate place in the hody of the work. Their citation here is riot to be regarded 

 as necessarily implying my acceptance of them. 



