360 FCETAL AND LARVAL DEVELOPMENT. 



must be classed in the same group as the Chaetopoda. The mesoblast (Vol. 

 II. p. 227) clearly originates as two bands of cells which grow inwards from 

 the blastopore, and in some forms (Paludina, Vol. II. fig. 107) become divided 

 into a splanchnic and somatic layer, with a body cavity between them. All 

 these processes are such as are, in other instances, admitted to indicate 

 Enteroccelous affinities. 



The subsequent conversion of the mesoblast elements into amoeboid cells, 

 out of which branched muscles are formed, is in my opinion simply due 

 to the envelopment of the soft Molluscan body within a hard shell. 



In addition to these instances I may point out that the distinction be- 

 tween the Pseudoccela and Enteroccela utterly breaks down in the case 

 of the Discophora, and the Hertwigs have made no serious attempt to 

 discuss the characters of this group in the light of their theory, and that the 

 derivation of the Echinoderm muscles from mesenchyme cells is a difficulty 

 which is very slightly treated. 



II. LARVAL FORMS: THEIR NATURE, ORIGIN AND AFFINITIES. 



Preliminary considerations. In a general way two types 

 of development may be distinguished, viz. a foetal type and a 

 larval type. In the foetal type animals undergo the whole or 

 nearly the whole of their development within the egg or within 

 the body of the parent, and are hatched in a condition closely 

 resembling the adult; and in the larval type they are born at an 

 earlier stage of development, in a condition differing to a greater 

 or less extent from the adult, and reach the adult state either 

 by a series of small steps, or by a more or less considerable 

 metamorphosis. 



The satisfactory application of embryological data to mor- 

 phology depends upon a knowledge of the extent to which the 

 record of ancestral history has been preserved in development. 

 Unless secondary changes intervened this record would be com- 

 plete ; it becomes therefore of the first importance to the 

 embryologist to study the nature and extent of the secondary 

 changes likely to occur in the foetal or the larval state. 



The principles which govern the perpetuation of variations 

 which occur in either the larval or the foetal state are the same 

 as those for the adult condition. Variations favourable to the 

 survival of the species are equally likely to be perpetuated, at 

 whatever period of life they occur, prior to the loss of the re- 

 productive powers. The possible nature and extent of the 



