ORIGIN OF THE GERMINAL LAYERS. 297 



derived from niesenchyme cells. That in many cases, and strikingly so in 

 the Chordata, the muscle cells retain clear indications of their primitive 

 origin from epithelial cells, I freely admit ; but I do not believe either 

 that its histological character can ever be conclusive as to the non- 

 epithelial origin of a muscle cell, or that its derivation in the embryo 

 from an indifferent amoeboid cell is any proof that it did not, to start 

 with, originate from an epithelial cell. 



I hold, as is clear from the preceding statements, that such immense 

 secondary modifications have taken place in the development of the meso- 

 blast, that no such definite conclusions can be deduced from its mode of 

 development as the Hei-twigs suppose. 



In support of the view that the early character of embryonic cells is 

 no safe index as to their phylogenetic origin, I would point to the few 

 following facts. 



(1) In the Porifera and many of the Co3lenterata (Eucope polystyla, 

 Geryonia, &c.) the hypoblasb (encloderm) originates from cells, which ac- 

 cording to the Hertwigs' views ought to be classed as mesenchyme. 



(2) In numerous instances muscles which have, phylogenetically, an 

 undoubted epithelial origin, are ontogenetically derived from cells which 

 ought to be classed as mesenchyme. The muscles of the head in all the 

 higher Vertebrate, in which the head cavities have disappeared, are 

 examples of this kind ; the muscles of many of the Tracheata, notably the 

 Araneina, must also be placed in the same category. 



(3) The Mollusca are considered by the Hertwigs to be typical Pseudo- 

 erela. A critical examination of the early development of the mesoblast 

 in these forms demonstrates however that with reference to the mesoblast 

 they must be classed in the same group as the Chsetopoda. The mesoblast 

 (Vol. I. p. 188) clearly originates as two bands of cells which grow 

 inwards from the blastopore, and in some forms (Paludina, Vol. I. 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 in- 

 stances, admitted to indicate Euterocoelous affinities. 



The subsequent conversion of the mesoblast elements into amreboid 

 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 110 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 develop- 

 ment, in a condition differing to a greater or less extent from the 



