52 THE DOCTRINE OF DESCENT. 



brane (Ectoderm, or Exoderm) and the internal mem- 

 brane (Entoderm). And this phase of the ciliated 

 larva, with its twofold strata, its primitive ventral cavity 

 and mouth, recurs in the Ccelenterata, with slight varia- 

 tions in the Echinoderms, in some of the Annulosa, in 

 the Sagitta, the Ascidians, and the Lancelet. From 

 the analogy of all these animals, and especially of the 

 last, we shall be able hereafter to derive important 

 inductions. 



But if no weight be attached to the presence of 

 these filaments of the external layer, which is, moreover, 

 justified by the relation of the filament to the cell, and 

 if it be acknowledged as the essential significance of 

 the larval arrangement, that from its two laminae the 

 collective organs derive their origin, then to the animals 

 above enumerated must be added, not only almost the 

 whole of the Articulata, but likewise the remainder of 

 the Vertebrata, as in them, immediately after the appear- 

 ance of the primitive striae, follows their separation into 

 two cell-layers, or membranes. Respecting the deriva- 

 tion of the third or middle germinal lamina, and the 

 share of the two primitive laminae in its formation, 

 observers are not agreed. 



Only from this point does the development of the great 

 animal groups take various directions, and it is the im- 

 mortal merit of Von Baer to have fixed these types of 

 development, independently of the fundamental forms, 

 established by Cuvier on zoological and anatomical con- 

 siderations, and he thereby laid a far deeper foundation 

 for the existence of these types. We will illustrate our 

 meaning by two examples. 



When the ovum of the articulate animal has sur- 



