204 Mr. A. Hyatt on the 



outgrowth, increasing continually the diameter of the amphi- 

 blastula, then invagination, then peripheral growth of the 

 ectoblast, followed by elongation of tlie planula and contrac- 

 tion or obliteration of the blastopore. 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 gastrulffi as well as for the existence of 

 the planula. The action of heredity 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 

 enclosed, 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 trans- 

 formed into this concentrated form of development through 

 the epibolic gastrula as an intermediate stage. 



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

 Basel, Georg, 1876) has given the fullest iUustrated account 

 of what we have, in common with Metschnikoff and Schultze, 

 called the transient gastrula of the Calcispongiaj. 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 tliese cases of transient invagination. It 

 is possible that the invagination stage may be traceable 

 directly to excess of growth in the ciliated cells and their 

 subsequent evagination as outgrowths to the reversal of this 

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

 this direction. 



We have also noted in our original essay the probability 

 that the medullary fold was primitively a stomodeal invagi- 

 nation due to extra growth, and we are able to quote in this 

 connexion an observation of Dr. Hatschek's in addition to 

 those of Kollmann and Gardiner. 



Dr. Hatschek (Arb. Zool. Inst. Wien, vol. iv. 1881, 

 pp. 45-48) attributes the origin of the primitive segments 

 and other changes of form in the embryo of Amphioxus to 

 the growth and energy of cells. He explains the origin of 

 the medullary plate by differentiations in the cells caused by 

 the extra growth of the neighbouring cells of the ectoderm. 



