DEVELOPMENT OF FLUSTRBLLA HISPIDA. 453 



now possible to distinguish between a longitudinal (long) and 

 a transverse (short) axis of the larva. At a later period^ 

 however, cell division is found to occur more rapidly along 

 the transverse than along the longitudinal axis, and the larva 

 assumes a spherical shape ; so that it is not therefore prac- 

 ticable to establish any direct correlation between the long 

 and shorfc axes of these early stages and the long and short 

 axes of the mature larva. 



The 16-cell Stage. — Each of the four large lower cells 

 next divides into two cells of unequal size. The plane of 

 cleavage is a vertical one lying more or less at right angles 

 to the long axis of the embryo, and the four central cells 

 formed are the larger; so that now the oral surface of the 

 larva also consists of two rows, each of four cells, which 

 immediately underlie those of the upper series. The larva 

 (PI. 23, figs. 47«-c) is now, therefore, built up of sixteen cells 

 arranged in four parallel rows, which are disposed in two 

 tiers ; that is, it consists of an upper, aboral tier of two rows, 

 each of four cells, and of a lower, oral tier of larger cells, 

 also of two rows, each of four cells. All of these sixteen cells 

 belong to the fifth generation (the unsegmented ovum being 

 regarded as the first generation). The four central cells of 

 the oral series are much larger than the lateral ones of the 

 same series, and these again are larger than those of the 

 upper tier. 



At this stage, the segmentation cavity {8.C.) becomes 

 visible, and it is noteworthy that it contains a substance 

 which stains feebly with eosin, safranin, borax-carmine, etc. 



It may be mentioned that the arrangement of the cells in 

 two tiers of two parallel rows each of four cells is charac- 

 teristic of many Polyzoan larvce at the 16-cell stage. 



The 20-cell Stage.— The next division (PI. 23, fig. 48) 

 also takes place in the four large central oral cells, which each 

 again divide into two unequal cells — A J and A |, B ^ 

 and B I, C J and C I, and D | and D | — cleavage taking 

 place in a vertical plane at right angles to the previous 

 division. The embryo (PL 23, figs. 49 a-c) now consists of 



