532 G. C. BOURNE. 



terates, is admitted; the existence of a mesoderm, if by it we 

 understand a tissue or aggregate of tissues interposed between 

 the two primary germ layers, is not denied, but it is specifi- 

 cally denied that those intermediate tissues originate from a 

 separate embryonic layer, composed of undifferentiated cells. 

 Ectoderm and endoderm, says the author, are the foundations 

 of all other tissues and organs, with the sole apparent exception 

 of the germinal cells, not only in Ccelenterata but in all 

 Metazoa. Tissues and organs originating directly from these 

 layers take over with them the power of giving origin to other 

 tissues and organs on their own account ; in no part of the 

 living body is the capacity for change entirely lost. By this 

 the genetic relationship between any organ and the primary 

 germ layers is not destroyed, it is only made more remote by 

 the intercalation of one or more intermediate developmental 

 phases. Say that an organ or tissue which originates directly 

 from one of the primary germ layers is a primary tissue; it 

 gives rise to other tissues which are secondary, those give rise 

 to another series of tertiary tissues, and so forth. The meso- 

 blast, so called, is in fact nothing more than the aggregate of 

 these primary, secondary, and tertiary tissues, the precise origin 

 of which is often obscured by the tendency to precocious 

 development. 



Let us see how this view is supported by the facts of the 

 development of a single form. The account of the develop- 

 mental phases of Lopadorhynchus is so long that it would be 

 impossible to give details of the whole organogeny of the 

 animal in the space of a short essay. I will therefore confine 

 myself to a short description of the general development of the 

 Annelid from the larval form, and will give the details of the 

 formation of a few selected tissues and organs which have a 

 special bearing on the question in hand. 



The earlier developmental history of Lopadorhynchus was 

 not studied ; the youngest larva described is a true trocho- 

 sphere, with a preoral ring of cilia, called by Kleinenberg by 

 the convenient name of prototroch, dividing the body into pre- 

 oral and oral lobes of nearly equal size. The stomodseum 



