40 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
glass to be moved upon the rollers, and the egg oriented as desired, even 
after the slide has stood for months. 
The method of embedding and orienting preparatory to it de- 
scribed by Dr. Woodworth (93) proved to be very valuable. The 
ordinary method of orientiug in melted parafine on the warm stage 
with the aid of a lens was also employed. Sections were cut 6.67 p 
in thickness, and reconstructions of many stages were made in wax 
on a scale of three hundred diameters. Transverse, sagittal, and frontal 
sections were cut; though it was not always possible to orient the 
embryo exactly, the reconstructions revealed the direction of the sections 
in cases where there was doubt. Sagittal sections are more readily 
interpreted than the others, for in them the cells of the different germ 
layers are shown in the same section in such relations as to be more 
easily recognized than in sections in other planes. 
In the discussion of sections the following orientation is used. The 
end called anterior is the one toward which the growing invagination is 
directed. At the time of gastrulation it is the larger end of the embryo. 
The opposite end is the posterior, and is marked at the stage preced- 
ing gastrulation by a greater thichness than the anterior end, duo to 
the presence of the mesoderm. In the early stages of gastrulation 
the broader and shallower end of the blastopore lies anterior. At the 
completion of gastrulation the contracted remnant of the blastopore 
occupies a terminal position at the posterior pole. "Phe chief axis is the 
antero-posterior one. The ventral surface is marked in the blastula by a 
greater convexity than the dorsal, but during the period of gastrulation 
by the growing invagination. Seetions are called sagittal that are 
parallel to the plane which coincides with both chief aud dorso-ventral 
axes; frontal, those that are perpendicular to the dorso-ventral axis; 
transverse, those that are perpendicular to the chief axis. 
111. NOMENCLATURE OF SPIRAL CLEAVAGE. 
The earliest full discussion of spiral cleavage occurs in Blochmann’s ad- 
mirable work upon Neritina (81). Fol (75 and ’76) had described the 
early stages in the cleavage of the Pteropods and the Heteropods, and 
Rabl (79) the cleavage of Planorbis ; but neither had entered into a full 
discussion of the lineage of the cells or the spiral character of the cleavages 
with which he was dealing. In Neritina the cleavage is unequal, and 
at the formation of the first set of micromeres we have the appearance of 
