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



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 ihe well known 

 case of Amphioxus described by Kowalevsky, and many ex- 

 amples by other authors. The growth and excess of pressure 

 is also evinced in the elongation of the planula and the ten- 

 dency of the at first broad blastopore to close up to a narrow 

 opening by growth of the ectoblast. The usually columnar as- 

 pect of the ectoblastic cells of the planula, their longest axes 

 being radial, or at right angles to the direction of the pressure, 

 is also favorable 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 mi- 

 croscope. We feel obliged to join those authors who regard 

 the planula stage as an abbreviated form of the gastrula possi- 

 bly directly derived from the epibolic gastrula. The succes- 

 sion of the stages is first a peripheral outgrowth increasing 

 continually the diameter of the amphiblastula, then invagina- 

 tion, then peripheral growth of the ectoblast, followed by elon- 

 gation of the planula and contraction or obliteration of the blas- 

 topore. Heredity in these cases seems to be subordinate to 

 growth, but this we think is due to the necessarily identical 

 action of these inseparable forces. Heredity and growth are 

 also necessary in order to account for cases of epibolic gastrulae, 

 as well as for the existence of the planula. The action of hered- 

 ity in the planula is obvious, but in the transitional epibolic 

 gastrula the obvious mechanical action of growth still interferes 

 with the clear perception of the influence of heredity. The 

 growth of the ectoblast cells is so rapid in the last named, that 

 the endoblast cells become inclosed as in the Ctenophoras, and 

 the gastrula is formed by a process much shorter than is usual 

 in embryos of the embolic type. 



In a planula we can see very clearly that some other force 

 in addition to growth has been at work, and that, whether we 

 adopt Lankester's hypothesis or some other, we are equally 

 obliged to call in the aid of heredity in order to explain the 

 hidden steps by which the embolic gastrula has been transformed 

 into this concentrated form of development through the epi- 

 bolic gastrula as an intermediate stage. 



Keller (Anat. und Entwickel. einiger Spong. d. Mittelmeers, 

 Basel, Georg, 1876), has given the fullest illustrated account of 

 what we have, in common with Metschnikoff and Schultze, 

 called the transient gastrula of the Calcispongias. A recent 

 perusal of this interesting paper has suggested that there is 



