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 Ceelenterata, 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 lamine 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 striz, 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 lamine 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- 
