A. Hyatt — Larval Theory of the Origin of Tissue. 341 



The Parenchymula is a recently discovered stage of the 

 embryo immediately succeeding the closed blastula. The eso- 

 teric cells differentiated during preceding stages have been 

 found, by several authors, to quit the exterior where they 

 originated and wander into the interior where they presumably 

 give rise to the endoblastic cells subsequently found there. 



A differentiated colony, like the amphiblastula with the cells 

 at one end becoming better fitted to take in food, could be trans- 

 formed into a parenchymula by the migration of differentiated 

 feeding cells into the interior and the Parenchymula could 

 thus have been transformed into a true gastrula. There are no 

 living forms, so far as we know, with wnich the Parenchymula 

 can be compared, and its probable meaning has already been 

 indicated hv other writers, especially by Metschnikoff, namely, 

 .that it implies a radical form in which the mesenchyme has 

 arisen as a primitive mass by delamination. 



The inwandering of the esoteric cells of the parenchymula 

 might be reasonably assumed as in part due to pressure. This 

 appears to be a primitive mode of forming the endoderm 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 possible that this tendency 

 was derived from ancestors in which a primitive invagination 

 appeared as a later characteristic of the development, due to 

 excess of growth in peripheral parts and that the same con- 

 ditions 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 invagin- 

 ation. The continued action of tne same cause as gave rise 

 to the tendency to inwandering, namely, the pressure occa- 

 sioned 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 de- 

 cendent forms. The formation of stomodea and other ectoder- 

 mic invaginations can thus be accounted for as in every way 

 parallel to formation of the gastrula, and due to similar causes. 



