M{sceU(ir)co}(s. 161 



an acute pole. The masses of ova are at first soft and compressible, 

 but the two sacs separate from the oviducts, fall into the cavity of 

 the shell, and gi'adually become converted into harder, flattened 

 masses. The greater firmness is to be ascribed to the increased 

 thickness of the vitelline envelopes of the individual ova, and to the 

 hardening of the material which binds thera together. This cement 

 miist become softened again when the embryos are ready to creep 

 out. 



The fecundation of the ova takes place before the formation of 

 the egg-sac. Living spermatosomata are occasionally found in the 

 sac. Before the ova have reached the end of the oviducts which 

 lead from the peduncle of the parent to the two sides of the head, 

 the vitellus is of uniform structure and permeated throughout with 

 lecythin-globules. On the separation of the directive bodies, 

 which takes place after the penetration of the male element, the 

 contents of the ovum are arranged so that at its rounded pole there 

 is a dome of finely granulated vitellus, while towards the acute pole 

 the lecythin-granules are collected together imbedded in a coarsely 

 granular substratum. The separation of the directive bodies 

 occurs at the obtuse ]X)le and is accompanied in the ovum of Lepas 

 by changes in the vitellus somewhat as described by the author in 

 the ovum of Ascaris megalocepliala. The two pronuclei are also in 

 the neighbourhood of the obtuse pole, with their surfaces of contact 

 and fusion perpendicular to the long axis of the ovum. 



The first division takes place, as in Ascaris nir/rovejiosct, perpen- 

 dicularly to the fusion-surfaces of the pronuclei, and therefore in the 

 long axis of the ovum. The plane of division produces two unequal 

 globules of segmentation ; the lecythin is contained only in the 

 larger one. Then occurs a turning of the segmentation-spheres and 

 a displacement of the contents of the nutritive cell, which culmi- 

 nates in bringing the surface of divisiou into the equator of the 

 ovum, perpendicular to its first direction. It is well known that in 

 segmentation the superior animal-cell precedes the inferior vegeta- 

 tive one and grows around it. As to the formation of the germ- 

 layers the information is imperfect ; in fresh specimens an invagi- 

 nation -gastrula appeared to be formed. The head of the larva is 

 always at the obtuse and the tail at the acute pole. 



The results of the investigation are summarized as follows : — 



The processes of maturation and fecundation of the ovum of Lepas 

 arranged the vital parts in such a way that with the separation of 

 the directive vesicles all the axes of the future embryo are already 

 defined. The separation of the directive corpuscles and the first 

 and second segmentations take place in the future long-axis of the 

 animal, and the position of the directive vesicles indicates the 

 future position of the cephalic portion of the embryo in course of 

 formation. 



If the relative positions of the axes continued in the way first 

 occurring, it might be imagined that the contents of the ovum 

 exclusively possessed the whole power of orientation. But as the 

 first plane of division passes from a longitudinal to an equatorial 

 plane, the envelope and its form must also possess directive 



