MACKEREL INVESTIGATIONS IN 1897. . H 



and the four blastomeres resulting will soon be found to have shifted 

 around the polar axis of the egg from left to right, supposing one to be 

 facing the external surface of the segmenting disk. Protoplasmic 

 movements, followed by a long resting stage, characterize this as well 

 as succeeding periods of development. 



The third and fourth furrows, parallel respectively to the first and 

 second, and resulting in the formation of 8 and 16 blastomeres, are 

 completed in 4 hours. Already, at this stage, irregularities in seg- 

 mentation begin to appear, and their frequency makes the mackerel 

 somewhat remarkable among fishes having similar eggs and modes of 

 development. Even in the 8-cell stage one of the blastomeres may be 

 much smaller than the others, or pushed from its place to a deeper 

 plane than its fellows. In the formation of the fourth cleavage one or 

 two of the cells may fail to divide with the others, producing a stage 

 with 15 or 14 cells, or the division may be unequal, and in nearly all 

 cases it is un symmetrical. In most teleostomous fishes such irregulari- 

 ties occur much less frequently and not so early in the developmental 

 process. The Spanish mackerel, however, presents a similar case. 

 Such irregularities may possibly indicate impaired vitality in the eggs 

 studied, but, as they occur with such frequency in every lot of material 

 examined, I am inclined to doubt this. In no case in which the future 

 development of such eggs was followed did the normal growth of the 

 blastodisc or embryo appear to be modified. The typical IC-cell stage 

 forms a nearly square figure consisting of 12 marginal and 4 central 

 cells, which latter have shifted spirally around the polar axis of the 

 egg as mentioned above. Up to this time the segmentation has been 

 perfectly rhythmical, occurring in all of the blastomeres at the saine 

 time, but, from now on, the central and marginal cells have a different 

 history, which, however, soon becomes confused. 



Following the 16-cell stage is one of 28 cells, resulting from the 

 nearly simultaneous division of the 12 marginal cells; those at the 

 angles of the square blastoderm dividing along planes approximately 

 diagonal; those at the middle of the sides along planes parallel with 

 the diameters. The central cells do not divide at this time, but soon 

 follow with a horizontal division, thus making two layers of cells in the 

 central part of the disk, while but one continues at its margin. Between 

 5 and 6 hours after the beginning of development the marginal cells 

 divide again, resulting in a disk containing about 56 cells. From this 

 time on the divisions occur in a more irregular way, though their rhyth- 

 mic character is still apparent. The marginal cells are for a time 

 smaller than the central ones, and begin to give rise to buds, in which 

 lie the outer ends of the division spindles, which give rise to the peri- 

 blastic nuclei. Succeeding the next division the cells of the entire 

 blastoderm become of very irregular shape, horizontal as well as verti- 

 cal cleavages are formed in all parts of the disk, and the marginal cells 

 have their longer axes placed in radial directions preparatory to the 

 rapid formation of the periblast nuclei and the superficial spread of the 

 blastoderm which now begins. 



