Pilidium of Certain N'emerteans. 239 



Cleavage. 



The eggs of many species of nemerteans will rarely be deposited in 

 confinement, but will develop readily when artificially fertilized. On 

 the southern coast of New England the eggs of Cerebratulus lacteus 

 may be thus fertilized during the months of March and April ; those 

 of C. leidyi, 3Iicritra cceca, Cephalothrix linearis, Eunemertes carcin- 

 ophila, and several species of Amphiporus and Tetrastemtna during 

 July and August. The eggs of Linens socialis are mature in mid- 

 winter, and those of L. viridis (gesserensis) from February to June. 

 At Naples an abundance of the eggs of Cerebratulus niarginatus may 

 be obtained in March and April. Those of the first three species, 

 C. lacteus, C. leidyi, 3f. cceca, as well as C. tnarginatus, develop 

 readily to pilidium-forms which are characteristic of the different 

 .species, and which will live for two weeks or more in confinement. 

 En none of these species, however, did I find it possible to keep the 

 pilidia alive until the development of the young nemertean. 



I The eggs of C. leidyi and M. cceca contain but little yolk, and 

 ^l-e beautifully transparent. Those of C. lacteus and C. marginatus, 

 /on the other hand, contain a large quantity of yolk, are larger in size, 

 and much less transparent. The pilidia, however, are equally trans- 

 parent in all. The phenomena of cleavage and gastrulation are very 

 similar in all four species, although the presence or absence of yolk 

 modifies them to some extent. 



It is, perhaps, advisable to follow the development in a single 

 species and compare the others with this. Let us choose for this 

 purpose Micrura cceca, the eggs of which fl^rnish an almost ideal 

 example of the regular, spiral type of cleavage. 



The ripe egg of this nemertean measures about .09'^°'" in diameter. 

 The vitelline membrane is extremely delicate, and can only be seen 

 at the point where the polar bodies are formed. It is therefore very 

 easily ruptured, and the polar bodies ai-e lost by even a slight disturb- 

 ance. The first cleavage takes place in the usual manner and divides 

 the egg into two apparently equal cells (Fig. 1, Plate xxxi). At a 

 temperature of 20° C. this occurs about one hour and ten minutes 

 after fertilization. The centrosomes of the first cleavage spindle 

 appear very early, as stated above, and the asters which are to form 

 the spindles of the second cleavage appear even before the comple- 

 tion of the first cleavage (cf. 8). The first cleavage usually passes 

 nearly through the point of extrusion of the polar bodies. A slight 

 " Zioischenkw'per^^ is formed, but this soon disappears. The two 

 blastomeres separate widely at first and remain connected only by a 



