i8 



FORMATION OF CELLS. 



[sect. 



II. 



around portions of contents, there are first formed in one cell several 

 nuclei from the original simple nucleus ; then the total contents of 

 the cell break up into exactly the same number of parts as there 

 are nuclei, one of them always enclosing a nucleus; and, lastly, 

 new membranes, the secondary cells, form around these portions of 

 contents (Nageli) or enveloping spheres (fig. 3.) To this category 

 belong the processes taking place during the development of the 

 seminal corpuscles of the Nematoidea (see Reichert in Mull. A rch. 

 1843), anc l especially during the cleavage of the yelk, a peculiar 

 process, which, occurring at the period of the first development in 

 the ova of most animals, is to be regarded as an introduction to 

 the formation of the primitive cells of the embryo, and because 

 the ovum has the signification of a simple cell, comes under the 

 idea of endogenous cell-formation. After the primitive nucleus 

 ^•3. of the ovular cell, the germi- 



nal vesicle, has disappeared 

 on fecundation, the granules 

 of the yelk no longer form 

 one compact heap as for- 

 merly, but become scattered 

 and fill up the whole ovular 



Three ova of Ascaris Nigrovenosa. 1. From the second, C - • J- hen there arises, as 



2. from the third, and 3, from the fifth stage of cleavage! flip fi r of -{„-,, ~f +],„ __ ¥V1 

 with two, four, and sixteen globules, a. External ovula Sr S1 S n 0t the COm- 



envelope b Cleavage globules. In 1. the nucleus of the menrino- rlpvplrmrnpnf o now 



lower globule, containing two nucleoli. In 2 the lower- lucncm g lieveiOpment, a new 



most giobuie, two nuclei. nucleus in the midst of the 



yelk around a new nucleolus. This is the first nucleus of the embryo, 

 which, acting as a point of attraction upon the yelk, again unites 

 it, to form a spherical heap In the further progress of develop- 

 ment, there are formed from the first nucleus, by endogenous 

 production, two new ones, which, as soon as they have become 

 free by the resolution of the mother-nucleus, separate somewhat 

 from one another, act as new centres upon the yelk granules, and 

 this the primitive heap breaks up into two. The multiplication 

 of nuclei and yelk segments (the former always preceding the 

 latter) then proceeds in a similar manner till a very large number 

 of small segments are present, filling up the whole space of the yelk 

 cells ; it is only exceptional that the yelk segments do not break up 

 until the nuclei have multiplied to three or four ; so that three or four 

 segments, instead of two, are directly formed from each of them 

 This process is called the total cleavage, because here the entire 

 yelk becomes arranged around the newly formed nuclei ; the par- 

 tial cleavage essentially agrees with it, only differing in the 



