EMBRYOLOGY Or CLliPSlNK. 283 



poda, !Fol (41) ). In Phascolosoma (i^f), and Vermetu?, and 

 Natica (ttt.ttttt)? it remains always in the entoderin-cells them- 

 selves. Lankester, in his valuable '^Contributions to the 

 Developmental History of the Mollusca/^ p. 18 and 25, has 

 shown the same contrast between Aplysia (major) and Pleuro- 

 branchidium, in respect to the position of the food-material, as 

 exists between Clepsine and Nephelis, or between Euaxes and 

 Lambricus (Kowalevsky). In Aplysia the food-material lies in 

 the endodermal sac, in Pleurobranchidium it lies outside the 

 same, in the form of two big cells which remain persistently 

 with their large pellucid nuclei and give rise to no pro- 

 geny. From these few examples it is apparent that the ultimate 

 place of the deutoplasm may be either (1) in, (2) outside, or (3) 

 inside the entoderm. The first position is undoubtedly the 

 original one, and the other two may be regarded as departures 

 from this, resulting from the increase of the passive food-yolk. 



This is in harmony with the fact that in some cases of unequal 

 cleavage the first position is followed by the third (Clepsine) or 

 the second (Lymnseus) ; while in other cases (Natica, &c.) the 

 first position is maintained throughout. 



{d) Free Nuclei. — As Leuckart ( uy ) long ago pointed out in his 

 paper on Melophagus, there is no essential difference between ordi- 

 nary segmentation and the formation of cells from free-formed nu- 

 clei. In one case the cleavage is simultaneous with the division of 

 the nucleus ; in the other it follows after a shorter or longer interval. 

 Passive yolk obstructs and, if increased beyond certain Hmits, 

 prevents cleavage. It is not, therefore, surprising to find in 

 eggs loaded with nutritive material the cleavage retarded or even 

 interrupted for a time while the nuclear activity is continuous. 

 The polar or peripheral segregation of the proper cleavage- 

 material, which becomes more and more marked with the accu- 

 mulation of deutoplasm, accounts for the simultaneous occurrence 

 of both modes of cell-formation, as in Clepsine, and numerous 

 other cases. 



The wide distribution of free nuclei formation in two of the 

 three secondary modes of cleavage {unequal and discoidal) is a 

 fact of late discovery. So far as I am aware Eay Lankester 

 (1873) was the first to recognise such phenomena in the eggs 

 of Mollusca (Lohgo, Octopus, and Sepia). His account of 

 these bodies which he termed " autoplasts," supposing them to 

 originate as independent segregations of the ''formative material," 

 leaves no doubt that they correspond to what I have called 

 '^ enioplasts''' {-ji^\-). '^Before the superficial extension of 

 the cap of klastoplasts (blastodisk) has commenced there appear 

 in a deeper stratum of yelk pellucid nuclei, at first arranged in a 

 circle around the cap of klastoplasts as I have figured them in 



