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 number becomes so great that individual 
identification is limited to special cases. However, by continued 
observation of successively developing stages one becomes familiar 
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 
with approximate if not absolute certainty. 
An examination of figs. 69 and 70 will show that 2b”? and 2b?!” 
have divided again, and shortly afterward cleavage occurs in a num- 
ber of other cells, 2b”, 2b74, etc. The upper cells of the third quartet 
in the anterior quadrants lie at first 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 will develop. The second quartet groups which lie laterally 
below the ends of the lateral arms of the cross also grow in extent and 
numbers, 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 
