432 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



TABUE I. Showing the number of nuclei in yolk, and the number of other " embryvit ic >i m-lei," and the 

 relative increase and decrease in these bodies from the close of yolk-segmentation to tltv eyy-nauplius 

 period. 



* Primary yolk cells. 



t Number obtained by method described bolow. 



The distribution of the wanderiug cells and of the embryonic uuclei is also illustrated by a 

 series of curves, constructed in the following manner : Each vertical row of the smaller squares 

 corresponds to a section of the ovum (Sections Nos. 1, 2, 3, etc.), while each square in this row 

 represents a single nucleus. The number of nuclei in a section is shown by the number on the 

 line where the curve makes a bend or intersects the middle of the lower side of a square (nuclei 

 Nos. 1, 2, 3, etc.). Thus where the descending line of the curve in Fig. 5 stops in section No. 2, 

 making an angle in the fifth vertical square, counting from the upper base line, this implies that 

 in the second section of the egg, corresponding to this vertical area, there were five nuclei which 

 did not appear in the following or preceding section. 



The number of nuclei were determined in the following way : Camera-lucida drawings were 

 made of every section of a given series on thin paper and each nucleus was marked. Then, by 

 superimposing upon each drawing the drawing of the section immediately following, every nucleus 

 new to that section could be determined in the early stages with absolute certainty. The number 

 of primary yolk cells and wandering cells were thus counted in all stages. In the older embryos 

 (see numbers marked with dagger in Table i, Stage m-v) where this method became impracti- 

 cable with reference to the total number of embryonic nuclei, their number was estimated in a 

 difierent manner. The uuclei appearing in each section were counted and the total number of 

 nuclear sections was thus obtained for the whole series. Then the percentage which the actual 

 number of nuclei in the egg bore to the total number which appear in sections could be determined 

 approximately by the method described above applied to a number of lateral sections, that is, by 

 actual count of uuclei in a favorable part of the series. The percentage which the actual number 

 of wandering cells bears to the total number appearing in section could be exactly determined and 

 compared with the former estimated percentage. The percentage which involved the wanderiug 

 cells was in the average of all stages a trifle the smaller, showing that the yolk cells were, on the 

 average, a little larger than the other embryonic nuclei. In Stage n before invagiuation the super- 

 ficial nuclei are the larger, while after invagiuatiou the difference is at first very slight indeed. 

 In Stages in and iv the wandering cells are markedly the largest in the egg, while in Stage v they 

 either equal or fall slightly below the size of the other embryonic nuclei. 



This method presupposes a perfect series of sections of uniform thickness. These conditions 

 were approximately fulfilled. The egg (i mm , or ^ inch in diameter) was cut, on the average, iuto 

 57 sections, each section being yir mm in thickness. The size of the egg, neither too small nor too 

 large, rendered this species (called throughout this paper the Bahaman variety of Alpheus heter- 

 most favorable for study so far as technical difficulties were involved. 



