SMALLWOOD : MATURATION OF HAMINEA SOLITARIA. 267 



of Conklin (:0l)/ have failed to trace back the centrosome to its earliest 

 condition. In most cases recorded, such as Byrnes ('99), Linville (:00), 

 Lillie ( :0l), and others, the earliest centrosomes found have been well 

 formed, have lain on opposite sides of the germinative vesicle, and have 

 been connected to each other by a well-formed central spindle, the 

 chromosomes of which lay in the equatorial plane, — the beginning of 

 the metaphase. 



In Haminea a distinction can be made between the centrosome (inclu- 

 sive of the centriole) and the differentiated cytoplasm immediately en- 

 veloping it. To the latter I shall apply the term cortical layer (st. ctx.) ; 

 but it is to be noted that this cortical layer becomes differentiated 

 into two zones, — a medullary zone (st. 7ned.) and a peripheral or cortical 

 zone (st. ctx.). These conditions will be discussed in the section "After 

 Deposition" (pp. 279, 280 ff.). 



The eggs of Haminea pass through several maturation changes while still 

 in the ovotestis. I have made a number of attempts to secure eggs at 

 the beginning of mitosis ; but in every animal examined after egg-laying 

 had begun, all of the ovarian eggs, even if they still remained in the 

 follicles, had a partly formed spindle extending through the germinative 

 vesicle with a definite centrosome at each pole. This made the getting 

 of early stages of maturation difficult. It is mere chance if one 

 kills an animal when the ova are at the beginning of their meta- 

 morphosis, for there is no external sign to indicate this epoch, such as 

 there is after the beginning of copulation. The advanced condition 

 following copulation is very regrettable. When we consider that the 

 living animals are not common, and that it is necessary after collecting 

 them and before killing them, first, to observe when they copulate, and 

 afterwards to watch until they show signs of laying, the difficulty of the 

 problem becomes evident. However, stages have been secured that are 

 much earlier than those usually observed in Mollusca. They must be 

 regarded, I think, as representing a very early phase of karyokinesis, 

 for the centrosome (c'so.) in the earliest stage observed is not a homo- 

 geneous single body ; it consists of at least five spherical masses placed 

 in close apposition (Plate 1, Fig. 4). The clear area that appears in the 

 centre of this group of bodies is due to their arrangement. This is shown 



1 Had Professor Conklin's most admirable monograph on " Karyokinesis and 

 Cytokinesis in the Maturation, Fertilization and Cleavage of Crepidula and other 

 Gasteropoda " (Jour. Acad. Nat. Sci. Philad. XII. 1902) appeared before this 

 article was sent to press, I should have attempted to include an extended com- 

 parison of Crepidula and Haminea. 



