1874.] Developmental History of the Mollusca. 235 



or ancestral mode of development, and what relation has the orifice of 

 invagination in the one case to the mouth which, later, breaks its way 

 through in the other ? 



It is not within the scope of the present memoir to discuss these ques- 

 tions at length ; but the author is of opinion that we must regard the 

 Gastrula-s&c with its endoderm and ectoderm as strictly equivalent (ho- 

 mogenous, to use another expression) in the two sets of cases. One of 

 the two methods is the typical or ancestral method of development, and 

 the departure from it in the other cases is due to some disturbing condi- 

 tion. He believes that we shall be able to make out that disturbing 

 element in the condition of 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 re- 

 lations of bulk in the early developed organs of the various embryos con- 

 sidered, determine the development either by invagination or by delami- 

 nation. The relation of bulk to the process of invagination may be illus- 

 trated from a fact established in the preceding communications. In 

 Loligo 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 

 Aplysia the smaller otocysts develop, each, by a simple vacuolation of 

 the epiblast without invagination. In Loligo the chief nerve-ganglia de- 

 velop by invagination of the epiblast, in Aplysia by simple thickening. 

 Again, in Vertebrata the nerve-cord develops by a long invagination of 

 the epiblast ; in Tubifex and Lumbricus the corresponding nerve-cord de- 

 velops by a thickening of the epiblast without any groove and canal .of 

 invagination. 



The bulkier structures in these cases are seen to develop by invagi- 

 nation, the smaller by direct segregation. Invagination therefore acts as 

 an economy of material, a hollow mass being produced instead of a solid 

 mass of the same extent. 



Tha't the presence of a quantity of deutoplasmic matter, or of a par- 

 tially assimilated mass of such matter, in the original egg is not accom- 

 panied by well-marked invagination of the blastosphere, while the absence 

 of much deutoplasm is the invariable characteristic of eggs which de- 

 velop a Gastrula by invagination, is shown by a comparison of Aplysia 

 and Loligo with Pisidium, and Limax, and of the Bird with the Batra- 

 chian. In some cases, such as Selenka has characterized by the term 

 " epiboly," it seems that the enclosure of the large yelk-mass by the over- 

 growth 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 Lumbricus present in this respect, 

 as described by Kowalevsky, tends very strongly to establish a transition. 



But the mode of development of the Gastrula of Geryonidae, described 

 with so much minuteness by Poll, which is obviously the same as that of 

 the Gastrulce of Spongiadse and most Hydroids, is clearly no masked case 



