4 DEVELOPMENT OF THE FROG'S EGG [Ch. I 



Each thread is split longitudinally into two parts. Two cen- 

 trosomes again appear. The chromatin-thread next breaks up 

 into six bent rods or chromosomes (Fig. 2, C). There is some 

 doubt as to the way in which the next change is brought 

 about. The account of vom Rath, which we follow here, seems 

 to be in harmony with the process that is known to take place 

 in some other forms during this period of development of the 

 germ-cells. It appears that the halves of each of the six bent 

 rods begin to separate from each other except at the ends 

 of the rods, where the halves remain united. Each rod is 

 in this way converted into a ring (Fig. 2, D). These rings 

 are often so bent on themselves that they form a loop. The 

 six chromatin-rings lie close to the periphery of the nucleus. 

 The rings contract and become smaller and thicker (Fig. 2, E), 

 This stage lasts but a short time and is succeeded by a stage 

 shown in Fig. 2, F, G. Out of each ring four star-like granules 

 are formed, the tetrad or " Vierer-gruppe." The four granules 

 of each set are closely held together by clear linin threads. If 

 each granule be counted as a distinct chromosome, then there 

 are present at this time six groups of four chromosomes each, or 

 twenty-four chromosomes. These twenty-four chromosomes be- 

 come attached to the fibres of the achromatic spindle (Fig. 2, H) 

 and arrange themselves into an equatorial double plate. Then 

 twelve of these granules united in pairs wander toward one pole 

 of the cell and twelve toward the other pole, and the division 

 of the cell takes place (Fig. 2, I). This process is spoken of as 

 the first maturation-division. Without passing into a resting- 

 stage, a second division of each cell follows (Fig. 2, J). A new 

 karyokinetic spindle is formed and the twelve chromosomes 

 are separated into two plates of six chromosomes each, which 

 go to their respective poles. Each of the two new cells con- 

 tains therefore only six chromosomes (Fig. 2, K). The number 

 of chromosomes is now reduced to half the normal 7iumber 

 present in the other cells of the body of the animal. Each of 

 the four cells formed by these two consecutive divisions of the 

 sperm-mother-cell then differentiates into a spermatozoon (Fig. 

 2, L). Each spermatozoon consists of three parts, — a head, a 

 middle piece, and a tail. The head is formed almost entirely 

 out of the nucleus of the j)arent-cell of the spermatozoon, as 



