1904.] NATURAL SCIENCES OF PHILADELPHIA. 357 



as one hundred and fifty cells present this quartet consists of but four 

 cells in each quadrant. 



Gastrulation. 



With the beginning of gastrulation, marked differences appear in 

 the cleavages of the quadrants and the radial symmetry of the egg as 

 a whole gives place to a more and more distinct bilaterality. In 

 the posterior region, particularly among the cells of the second quartet, 

 great divisional activity and growth takes place; while the same series 

 in A, C and B quadrants show relatively slight increase when compared 

 with the derivatives of 2d. It has been impossible to follow the line- 

 age, except in particular instances, from the time these cleavages 

 begin, as most of the cells of the gastrula of Fiona are so similar in 

 size and appearance and the nvimber becomes so great that individual 

 identification is limited to special cases. However, by continued 

 observation of successively developing stages one becomes famihar 

 with the cell groups which will later give rise to various organs and, 

 aided by a few landmarks, may in most cases follow the organogeny 

 Avith approximate if not absolute certainty. 



.'^ji examination of figs. 69 and 70 will show that 2bi=i- and 2b2"2 

 have divided again, and shortly afterward cleavage occurs in a num- 

 ber of other cells, 2b2-, 2b-"\, etc. The upper cells of the tliird quartet 

 in the anterior quadrants lie at fii'st well toward the upper surface, 

 but as invagination proceeds these move around toward the lower side, 

 while an increasing number of second quartet elements are found sepa- 

 rating the first from the third quartet at the anterior as well as the 

 posterior end of the gastrula. Meanwhile the second quartet cells 

 in the median posterior region (derivatives of 2d) have multiplied very 

 rapidly, and by causing increase in the surface area of the gastrula in 

 this region have pushed the apical pole several degrees forward. Not 

 only have the posterior second quartet cells increased in numbers but 

 also in size, marking out at an early period the region from which the 

 shell gland vdW develop. The second quartet groups which he laterally 

 below the ends of the lateral arms of the cross also grow in extent and 

 nimibers, this being more particularly true of those which abut upon 

 the enlarging cells of the same series in D quadrant. 



The history of the third quartet has thus far been followed to a stage 

 when its members in each anterior quadrant number six, of which 

 four are large and two small cells, while in each posterior quadrant the 

 group comprises seven cells, three of which are small and four large. 

 By approximately horizontal cleavages of the upper cells in the two 



