PRODUCTION OF CELLS. xv ji 



The formation of cells by segmentation of the yelk may be traced with comparative 

 ease in the ova of many invertebrata. The accompanying figures represent the several 

 stages of the process in small species of the ascaris worm. Figures A, B, and o are 

 from the Ascaris nigrovenosa, as observed by Kolliker. He found that, after the 

 germinal vesicle had disappeared, a new nucleus with nucleolus was formed in its 

 place. The segmentation then goes on as in the mammalian ovum, but the nuclei 

 are visible from the first ; and from appearances, such as those seen in the lowermost 

 segments of A and B, it seems probable that the division of the nucleus is preceded by 

 that of the nucleolus. 



I may remark that I once observed the ovum of the ascaris under the microscope 

 whilst one of the large segments was actually undergoing division. There was first a 

 very obvious heaving motion among the granules throughout the whole mass ; then 

 ensued a constriction at the circumference, which, proceeding inwards, soon com- 

 pleted the division ; but ail this time the nuclei were quite hidden by the enveloping 

 granular matter. 



In some animals the segmentation process affects only a part of the yelk. 



6. Division of free Cells. Essentially the same process of fissiparous 

 propagation by which the cells are multiplied within the ovum continues to 

 take place in the embryonic cells at future 

 stages, and in their descendants in the pig j^ 



organs and fluids of the body throughout 

 life. The steps of the operation have been 

 best observed and proved in the colourless 

 blood-corpuscles, but it takes place in various 

 other cells (fig. XL). 



1. Into two. The body of the cell is ob- 

 served to be somewhat lengthened, and its 



nucleus divided into two ; a constriction then Fig. XI. COMMENCEMENT, Pno- 

 begins in the middle and proceeds until the GRESS, AND RESULT, or Divi- 

 substance of the cell is parted into two halves, 

 each of which contains a nucleus. There can 



be little doubt that here also the division begins with the nucleus. The 

 colourless blood-corpuscle we presume to have no proper cell-wall, but if, 

 as some believe, an envelope or primordial utricle is present, it is also 

 involved in the division. 



2. Into more than two. The division is usually into two, as above 

 described, but Remak has observed instances in the frog larva of cells 

 dividing into as many as five or six new cells ; that is, after the nucleus 

 had divided in a corresponding way. The difference in this modification 

 of the process may simply be that whilst in the preceding case the two 

 halves of the divided cell part from each other, and the resulting new cells 

 undergo fresh subdivision solitarily, they in the present case remain asso- 

 ciated in a group. Perhaps the formation of pus-corpuscles from connective 

 tissue-corpuscles, as described by Virchow, may be an instance of this 

 kind. Where the swollen primitive corpuscle acquires a distinct cell- 

 membrane or capsule which incloses its progeny, the case comes to resemble 

 the so-called " endogenous " propagation, as in cartilage, to be next 

 described. 



c. Division of inclosed Cells An example of this is afforded by carti- 

 lage. The cells in that tissue are surrounded by an outer capsule, which 

 is continuous, or at least coherent, with the substance of the matrix 

 (fig. xii. , A). Here, as in the previous cases, there is first a division of 

 the nucleus into two; this is followed by cleaving of the cell-body like- 

 wise into two, and thus two young cells are formed from a parent one. 



b 



