I GROWTH OF THE GERM-CELLS 11 



and sense-plates are developed at the sides of the head, 

 a stomodaeum is pushed in in front, a proctodaeum behind, 

 and the head becomes separated by a slight constriction from 

 the neck. The tail grows out above the proctodaeum, the 

 external gills appear, the olfactory pit, eyes, auditory vesicles, 

 heart, gill-slits, coelom. pronephros are developed, and the 

 embryo is ready to hatch out as the tadpole. We need follow 

 the development no farther, but enough has been said to 

 illustrate the occurrence of the three processes of growth, 

 cell- and nuclear division, and differentiation. 



The third example will be one of those cases in which it is 

 possible to trace back each organ or system of organs in the 

 body to a particular cell or cell-group in the segmenting 

 ovum, to state in fact its cell-lineage. This is the round 

 worm, Ascaris megalocephala. The egg of Ascaris is very 

 small and spherical. In it may be distinguished, in addition 

 to the cytoplasm, some yolk globules (specifically lighter than 

 the cytoplasm) massed together on one side : the arrangement 

 is therefore telolecithal and the symmetry similar to that of 

 the frog's egg. There are also some spherules of a clear 

 substance, placed in the animal region. 



The first division (Fig. 5) is equatorial, separating an animal 

 cell AB or 8 l from a vegetative cell P r When the spindles 

 appear for the second division, it is seen that in the cell S l 

 the spindle axis is placed at right angles to the egg-axis, while 

 in P 2 the spindle lies in the egg-axis. S l is therefore about 

 to divide meridionally, P l latitudinally, and when the division 

 has been accomplished, the cells A and B resulting from the 

 division of S l and S 2 or E M St and P 2 resulting from the divi- 

 sion of P l , are arranged in a characteristic T shape, the cell P 2 

 being nearest the vegetative pole at the bottom of the stem 

 of the T. 



The very remarkable phenomenon of the diminution of the 

 chromosomes can at this stage (metaphase of the mitosis 

 prior to the division) be seen in the cell $j or AB. As the 

 four Y-shaped chromosomes lie in the equator of the spindle 

 the middle portion of each becomes transversely divided into 

 a row of small granules, while the ends are swollen. The 



