HETEROTOPY AND HETEROCHRONY. 1 3 



" Displacement in position, " or " Heterotopy," especially 

 affects the cells or elementary parts which compose the 

 organs; but it also affects the organs themselves. For 

 example, the sexual organs of the human embryo, as well as 

 those of many higher animals, appear to originate from 

 the middle germ-layer. But the comparative Ontogeny of 

 the lower animals shows, on the other hand, that these 

 organs did not originally arise from this layer, but from one 

 of the primary germ-layers ; the male sexual organs from 

 the outer germ-layer, the female from the inner. Gradually, 

 however, the germ-cells have altered their original site, and 

 have made their way, at an early period, from their original 

 position into the middle germ-layer, so that they now 

 appear actually to originate in the latter. An analogous 

 heterotopism affects the primitive kidneys in the higher 

 Vertebrates. Even the appearance of the mesoderm itself 

 is very greatly affected by a displacement in position, which 

 is connected with the transition of embryo cells from one 

 germ-layer into another. 



The kenogenetic " displacements in time," or " Hetero- 



chronisms," are equally significant. They are seen in the 



fact that in the germ-history (Ontogeny) the sequence in 



which the organs appears difiers from that which, judging 



from the tribal history (Phylogeny), would be expected. By 



heterotopy the sequence in position is vitiated ; similarly, 



I by heterochrony the sequence in time is vitiated. This 



! vitiation may effect either an acceleration or a retardation 



' in the appearance of the organs. We must regard the 



following incidents in the germ-history of man as examples 



j of ontogenetic acceleration : the early appearance of the 



i heart, the gill-openings, the brain, the eyes, the chorda, 



