ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 369 



(9) The process of conjugation exhibits two typical intraoval courses, first 

 an approximately radial course wliicli leads tlie sperm from the point of 

 penetration on the black margin, deep into the ovum, to the " nuclear layer " 

 of the yolk, and second, a nucleopetal course which conducts the two 

 nuclei towards one another within the " nuclear layer," but especially the 

 sperm nucleus to the female. 



(b) When the ovum is forcibly fixed with the axis oblique. (10) If the 

 inclination be but slight (20-30°), the above statements often hold good. 

 (11) The yolk-material is disposed in such a way that it lies symmetrically 

 in relation to the first plane of division determined by the direction of 

 conjugation. (12) If the inclination be greater, the influence of gravity on 

 the yolk produces a symmetrical disposition of the different material, and 

 this influences the first division in such a way that the plane of division is 

 definitely disposed to the plane of symmetry, either lying in it or at right 

 angles to it. (13) Here also the fixst nuclear division apparently occurs in 

 the direction of the conjugation of the nuclei. (14) The position of the 

 germinal vesicle is influenced by the obliquity of the axis, and the course of 

 the sperm by the streaming of the yolk in such a way that the conjugation 

 must often occur in a direction approximately transverse to the plane of 

 symmetry in the oblique ovum. Thus there results a frequent approxi- 

 mately transverse direction of the first segmentation, (15) But since the 

 first segmentation is most frequently either quite transverse to the plane of 

 symmetry or exactly in the same direction, it is necessary to suppose a 

 twisting action of the symmetrically disposed yolk on the segmentation 

 nucleus, during or after conjugation. This turning may be supposed to 

 occur so that the segmentation nucleus and its direction of conjugation 

 become either parallel or perpendicular to the plane of symmetry, accord- 

 ing as the line of conjugation is nearer one or other. (16) If the segmen- 

 tation nucleus and line of conjugation be turned into the plane of yolk 

 symmetry, the first nuclear division separates the material of the two 

 antimeres of the embryo, and the first plane of yolk division is the median 

 plane of the embryo. (17) If the turning result in a position perpendicular 

 to the above, the first nuclear division separates the nuclear material, as in 

 a normal second segmentation, into what ultimately become ventrocaudal 

 and dorsocephalic portions. (18) With greater obliquity of egg axis the 

 side of the depressed black pole becomes always the ventrocaudal side of 

 the embryo. With slight obliquity this tendency may conflict with that 

 determined by the position of fertilization (cf. 4 and 8), and the fertilized 

 side become the ventrocaudal, but this is only if the rearrangement of the 

 yolk occur in such a way that, at the time of second segmentation, the egg 

 axis has its black pole inclined towards the sperm. (19) The primary 

 cause for the position of the ventrocaudal surface on the side to which the 

 upper end of the egg axis is inclined is probably the accumulation of forma- 

 tive yolk on that side. 



Origin of Segmental Duct.* — Prof. A. C. Haddon summarizes the 

 history of the discovery of the epiblastic origin of the primitive duct of the 

 vertebrate excretory organ, and alludes to the difficulties which have 

 hitherto beset the interpretation of its morphology. 



Accepting the proposition that in primitive Chordata the nephridia were 

 segmentally arranged and opened directly to the exterior, we have only to 

 assume that the lateral area along which they opened was grooved and that 

 this groove extended posteriorly as far as the anus. From the analogy of 

 the neural groove, there is no great difficulty in further supposing that the 



* Proc. R. Dublin Soc, v. (1887) pp. 463-72 (1 pi.). 



