84 Royal Society : — 



equivalent (homogenous, to use another expression) in the two 

 sets of eas(>s. One of the two methods is the typical or ancestral 

 method of development, and the departure from it in the other case 

 is due to some disturbing condition. He believes that we shall 

 be able to make out that disturbing element in the condition o£ 

 the egg itself as laid, in the presence in that egg of a greater or 

 less amount of the adventitious nutritive material which Edouard 

 van Beneden calls " deutoplasm." This and certain relations of 

 bulk in the early de\"eloped organs of the various embryos con- 

 sidered, determine the development either by invagination or by 

 delamination. The relation of bulk to the process of invagination 

 may be illustrated from a fact established in the preceding com- 

 munications. In Lolufo the large otocysts develop, each, by a 

 well-marked invagination of the epiblast, forming a deep pit which 

 becomes the cavity of the cyst. In Aphj'iia the smaller otocysts 

 develop, each, by a simple vacuolation of the epiblast without in- 

 vagination. In Lolifjo the chief nerve-ganglia develop by invagi- 

 nation of the epiblast, in Aphjsia by simple thickening. Again, 

 in Yertebrata the nerve-cord develops by a long invagination of 

 the epiblast ; in Tuhifex and Lvmhrhus the corresponding nerve- 

 cord develops by a thickening of the epiblast without any groove 

 and canal of invagination. 



The bulkier structures in these cases are seen to develop by 

 inA'agination, the smaller by dii'ect gegregation. Invagination 

 therefore acts as an economy of material, a hollow mass being 

 produced instead of a solid mass of the same extent. 



That the presence of a quantity of deutoplasmic matter, or of 

 a partially assimilated mass of such matter, in the original egg is 

 not accompanied by well-marked invagination of the blastosphere, 

 while the absence of much deutoplasm is the invariable character- 

 istic of eggs which develop a Gastnda by invagination, is shown 

 by a comparison of Apli/sia and Loligo with Pisidium and Limax, 

 and of the Bird ^-ith the Batrachian. In some cases, such as 

 Selenka has characterized by the term " epiboly,"' it seems that the 

 enclosure of the large yelk-mass by the overgrowth of cleavage- 

 cells may be held as equivalent to the invagination of the large 

 yelk-cells by " emboly ; " and the intermediate character which 

 the development of Euaxes and Lxmhricus presents in this respect, 

 as described by Kowalevsky, tends very strongly to establish a 

 transition. 



But the mode of development of the Oastrula of Geryonida?, 

 descrilied with so much minuteness by Foil, which is obviously the 

 same as that of the Gastrulcp of Spongiadae and most Hydroids, is 

 clearly no masked case of invagination. There is no question of 

 " epiboly " here, but a direct and simple splitting of one cell into 

 two ; so that what was a sac formed by a layer of cells one deep, 

 becomes a sac formed by a layer of cells two deep, or of two 

 layers each one deep. 



It is yet a question for much further inquiry as to how this 

 mode of forming a double-walled Gastnda can be derived from, or 



