284 THE EVOLUTIONARY CAREER 



Vertebrates and lamellibranch molluscs are therefore not closely 

 related to each other in classifications, in spite of the resemblances 

 of some eyes in the latter group to eyes in the former one. 



94. ON THE PRIMARY HOMOLOGIES 



i. All nucleated cells are homologous structures. We make this 

 statement, although we cannot apply the criterion of homology 

 to nucleated cells — the history of which is unknown. But most 

 nucleated cells are very much alike, in that they divide (mitotic- 

 ally) in the same, very special way. They are capable, as single 

 cells, in appropriate conditions, of independent existence and of 

 reproduction by mitosis. (Tissue-cultures in artificial media 

 demonstrate this in very many cases.) 



ii. A hlastula-phase occurs in the emhryogenies of most multi- 

 cellular animals and this phase is homologous zvherever it occurs. 

 In very many cases, drawn from very many difll'erent animal 

 groups the blastula is typical in mode of origin and form. When 

 it is not typical the deviations can be accounted for, say, by the 

 influence of food-yolk in the segmentations, or by precocity of 

 potencies in the blastomeres. It is evident that animal emhryo- 

 genies are " constrained," by something in the physico-chemical 

 conditions, to take a certain typical course — that resulting in the 

 formation of the blastular form. 



Hi. All gastnda-embryos and phases in the development of 

 multicellular animals are homologous. The essential features in 

 the process of a gastrulation are these : {a) the surface is increased 

 by the invagination ; (h) Cells with diff'erent potencies are 

 segregated as endoderm and ectoderm or such segregation confers 

 different potencies on the cells ; (c) an internal, digestive cavity 

 is formed by the invagination of part of the blastular wall so as to 

 form the archenteron. 



In spite of deviations from type due to yolk in the ovum, or 

 to segregation of potencies in the ovum before cleavage occurs, 

 gastrulation is the same fundamental process in most animal 

 emhryogenies. Therefore we conclude that all gastrula-phases 

 are homologous and that the primary embryogenic cell-layers, 

 endoderm and ectoderm, are everywhere homologous. 



iv. The three germ-layers are homologous in all embryogenies. 

 There are different ways in which the mesoderm originates — it 



