1884.1 155 [Hyatt. 



oteric or endoblast cells of the parenchymula might be reasonably 

 assumed as in part due to pressure. This appears to be a primi- 

 tive mode of forming the encloderm, as stated by Schmidt and 

 Metschnikoff, and, therefore, we should have to consider pressure 

 as simply a possible cause aiding the tendency to inwandering, 

 as it appears in the habits of these cells of the parenchymula. It 

 is probable that this tendency was derived from ancestors in which 

 a primitive invagination appeared as a late characteristic of the 

 development, due to excess of growth in peripheral parts, as sug- 

 gested above in the stomodea of Porifera, and that the same con- 

 ditions of growth and pressure would continue to be present in the 

 similar parts in the young of descendent forms as long as the con- 

 ditions and habits were sufficiently similar and did not interfere 

 with hereditary tendencies. The fact that the esoteric hemi- 

 sphere is an excessive peripheral outgrowth of the exoteric cells 

 in the amphiblastula is in perfect accord with the successive stages 

 in the development of pits, and minor invaginations of the ecto- 

 derm. These are universally in their primitive stages peripheral 

 outgrowths of the outer membranes, which form simple hollows 

 and then these cups become hereditary invaginations in the em- 

 bryos of descendent forms. The formation of stomodea and other 

 ectodermic invaginations can thus be accounted for as in every way 

 parallel to formation of gastrulae, and due to similar causes. 



The invagination of the endoblast in the ordinary form of the 

 gastrula is immediately accompanied and caused, as stated above, 

 by pressure arising from the excess of growth in ectoblastic cells. 

 The pressure on the endoblast after invagination is shown by the 

 forms of the cells, which become elongated 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 are 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 long- 

 est axes being radial, or at right angles to the direction of the 

 pressure is also favorable to this theory. These cells may be at- 

 tenuated in Porifera at this stage (Barrois, Epong. de la Manche) 

 so as to assume an almost linear aspect under low powers of the 

 microscope. The succession of the stages is first a peripheral out- 



