ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 221 



(1) Method. — No patliological condition occurs, as the result of any 

 mode of preservation, in ova whicli have passed beyond the stage with 

 vesicular pronuclei. Boveri's principal methods of fixing vs^ere (a) by 

 alcohol of various degrees of concentration, with 1 per cent, acetic acid, 

 or (h) by picro-acetic acid. 



(2) The spermatozoon from its entrance to the extrusion of the second 

 polar globule. — There is no special " bouchon d'impregnation " nor 

 micropyle of Meissner. Polyspermy is exceedingly rare, and due to a 

 weakness in the ovum, which prevents it excluding other spermatozoa 

 after one has been received. It is in the highest degree probable that 

 the nucleus of the sperm in all stages consists of two independent 

 chromatin elements (Carnoy's type), except in those males which cor- 

 respond to the females with eggs including only one chromatin element 

 (van Beneden's type). These two varieties have to be distinguished. 

 Carnoy's type of ovum (with two elements) is always fertilized by a 

 sperm with two elements, and so with the other variety. 



(3) Nuclei of ovum and sperm till the formation of the first spindle of 

 division. — After the extrusion of the second polar body, the male and 

 female nuclei present very close resemblance. From the second polar 

 figure the female pronucleus receives the two chromatin elements 

 (Carnoy's type is followed) of the internal daughter-platOo These 

 become surrounded by homogeneous nuclear sap, from which the proto- 

 plasm is separated by a delicate membrane. Into these vacuoles, towards 

 the membrane, the chromatin-rods give off processes, which grow, and 

 grow together, till a framework is formed, in which the rod is lost. For 

 a while the results of the modification of each rod are separable, later 

 on this cannot be demonstrated. Minute nuclear bodies appear and are 

 distributed throughout the nucleus. The movements associated with 

 the making of the above reticulum are extremely Rhizopod-like. The 

 differences between the above account of the difierentiation of the 

 nucleus and that given by van Beneden depend upon the preservation of 

 the ova. 



In the sperm-nucleus also, a growing vacuole forms round the 

 chromatin elements, into this the elements give off anastomosing pro- 

 cesses, nucleolus-like bodies appear, the solid chromatin masses are gra- 

 dually transformed into framework, which is gradually drawn towards 

 the nuclear membrane. The chromatin elements, however, which form 

 the female pronucleus are very simple and regular, both in form and 

 disposition ; the opposite is true of the elements of the sperm ; and this 

 causes certain differences. 



The position of the nuclei within the ovum is then discussed ; van 

 Beneden's results as to the protoplasmic mantle of the sperm-nucleus 

 are corroborated and extended ; the " germ-dualism " theory of Zacharias, 

 involving the conclusion that the pronuclei of van Beneden are not 

 pronuclei, but already conjugated nuclei, is unfavourably criticized. 

 Boveri maintains the generally accepted view, which van Beneden now 

 also allows, that the two vesicular pronuclei really fuse into a uniform 

 segmentation nucleus. Details of this are given. Against van Beneden 

 and Zacharias the author contends that the nuclear filaments are not 

 necklace-like, but parallel in contour, homogeneous, and uniformly 

 chromatic. Nor has he ever seen the continuous coil they describe. 



(4) The changes in the cell-substance during this period. — Beyond 

 their four essential chromatin loops, the two nuclei furnish nothing for 



