Experimental Studies on Germinal Localization. 201 



of comparison. To facilitate the comparison I shall employ 

 Robert's nomenclature, which combines certain advantageous 

 modifications, suggested by Conklln, Mead and Child, of the sys- 

 tem I used In 1892 in describing the cell-lineage of Nereis. The 

 primary quadrants are designated as A, B, C and D (D being the 

 posterior one), the corresponding micromeres as a, b, c and d; 

 the coefficient (i, 2, 3 or 4) designates the number of the quar- 

 tet, or In case of the basals (macromeres) the number of divi- 

 sions they have undergone; each exponent denotes a subsequent 

 division, i designating the cell nearer the animal pole, 2 the sis- 

 ter-cell nearer the "lower pole. Thus, starting with the 4-cell 

 stage, D divides Into iD and id; iD into 2D below and 2d 

 above; id into id^ above and id' below (the primary trocho- 

 blast) ; id^ Into id^-^ above (primary rosette-cell at the upper 

 pole) and id^, •- below (primary cross-cell) ; 2D Into 3D and 3d; 

 2d Into 2d^ and 2d", etc. Since in Patella the quadrants cannot 

 be distinguished by simple inspection before the 32-cell stage I 

 shall in general, where the quadrant is unknown, omit the letter. 

 Thus the primary trochoblast is i", the primary rosette-cell i^-\ 

 a primary quartet-cell i, 2, 3 or 4, and so on. 



Both Patella and Dentalium are typical examples of the spiral 

 type of cleavage, the former being of the symmetrical type (like 

 Crepidiila, Trochus, Hydroides or Polygordius) In which the 

 four quadrants are of nearly or quite equal size, the latter of the 

 asymmetrical type (like Nassa, Ilyanassa, Unio, Nereis or Jm- 

 phitrite) In which the first division Is unequal and the posterior 

 quadrant Is larger than the others until after both have been 

 formed. Dentalium, further. Is characterized by the formation 

 during the first three cleavages of a large polar lobe which after- 

 wards fuses with the posterior cell, CD, the egg passing at the 

 first cleavage through the characteristic "trefoil stage" that so 

 commonly occurs among mollusks (Nassa, Ostrea, etc.) and oc- 

 casionally In annelids (Myzostoma, Sahellaria, Chaetopterus) . 

 In a preceding paper ('04) I have sketched the early cleavage of 

 Dentalium and will here describe primarily that of Patella. 



The egg of Patella first divides Into equal quadrants, without 

 the formation of a polar lobe; and the 4-cell stage Is remarkable 



