Hyatt.] 156 [March 5, 



growth increasing continually the diameter of the amphimorula, 

 then invagination, then peripheral growth of the ectoblast, fol- 

 lowed by elongation of the planula and contraction or obliteration 

 of the blastopore. Heredity in these cases seems to be subordi- 

 nate to growth, but this we think is due to the necessarily identi- 

 cal 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 gas- 

 trula 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 Ctenophorae, 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 Irypothesis 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 trans- 

 formed into this concentrated form of development. Keller (Anat. 

 und Entwickelung einiger Spong. d. Mittelmeers, Basel, 1876), 

 has given the fullest illustrated account of what we, in common 

 with Metschnikoff and Schultze, have called the transient gastrula 

 of the Calcispongiae. 1 A recent perusal of this interesting paper 

 has suggested, that there is probably no better field for the study 

 of the effects of pressure upon cells, than in these cases of tran- 

 sient invagination. It is possible that the invagination stage may 

 be traceable directly to excess of growth in the ciliated cells, and 

 their subsequent invagination as outgrowths to the reversal of this 

 process, and at any rate this field is a very promising one in this 

 direction. We have also noted above, the probability that the me- 

 dullary fold was primitively a stomocleal invagination due to extra 

 growth, and we are able to quote in this connection an observa- 

 tion of Dr. Hatscheck's, which appears to bear out this supposition 



1 We entirely failed through an unlucky oversight, to quote this paper above, where 

 it would have been more appropriate. This is especially regrettable, since Keller's 

 observations on the possible entodermal origin of the primitive spicules, and the as- 

 sertion that the transient gastrula occurs normally after and not before the larva has 

 left the parent, and that the young larva in the early part of the ascula stage is without 

 cloaca, are exceedingly interesting in connection with the questions we have discussed. 



