Larval Theory of the Origin of Tissue. 203 



tendency was derived .from ancestors in which a primitive 

 invagination appeared as a later characteristic of the develop- 

 ment, due to excess of growth in peripheral parts, and that 

 the same conditions of growth and pressure would continue 

 to be present in the similar parts of the young of descendent 

 forms as long as the surroundings and habits were sufficiently 

 similar and did not interfere with hereditary tendencies. Thus 

 we should have to regard the habit of inwandering of the 

 esoteric cells as giving rise to the primitive endoblast, and 

 this last as a permanent stage preceding the transient gastrula 

 due to invagination. The continued action of the same cause 

 as gave rise to the tendency to inwandering, namely the 

 pressure occasioned by the rapid multiplication of external 

 cells by growth, and the action of heredity, would secure this 

 result. 



The fact that the esoteric hemisphere is an excessive peri- 

 pheral outgrowth of cells in the amphiblastula is in perfect 

 accord with the successive stages in the development of pits 

 and minor invaginations of the ectoderm. These are univer- 

 sally in their primitive stages peripheral outgrowths of the 

 outer membranes, which form primitive hollows, and then 

 these cups become hereditary invaginations in the embryos of 

 descendent forms. The formation of stomodea and other 

 ectodermic invaginations can thus be accounted for as in every 

 way parallel to formation of the gastrula and due to similar 

 causes. 



The invagination of the endoblast in the ordinary form of 

 the gastrula is immediately accompanied and caused, according 

 to Whitman, by pressure arising from the unequal growth of 

 the hemispheres. The pressure on the endoblast after invagi- 

 nation is shown by the forms of the cells, which become elon- 

 gated along the middle part of the cup, as in the well-known 

 case of Amphioxus described by Kowalevsky and many 

 examples by other authors. The growth and excess of 

 pressure is also evinced in the elongation of the planula and 

 the tendency of the at first broad blastopore to close up to a 

 narrow opening by growth of the ectoblast. The usually 

 columnar aspect of the ectoblastic cells of the planula, their 

 longest axes being radial or at right angles to the direction of 

 the pressure, is also favourable to this theory. These cells 

 may be attenuated in Porifera at this stage (Barrois, Epong. 

 de la Manche), so as to assume an almost linear aspect under 

 low powers of the microscope. We feel obliged to join those 

 authors who regard the planula-stage as an abbreviated form 

 of the gastrula possibly directly derived from the epibolic 

 gastrula. The succession of the stages is first a periphei-al 



14* 



