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 mouths. 



The method of embedding and orienting preparatory to it de- 

 scribed by Dr. Wood worth ('93) proved to be very valuable. The 

 ordinary method of orienting in melted parafine on the warm stage 

 with the aid of a lens was also employed. Sections were cut 6.67 ft. 

 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 reconstructious 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 discussioa 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 thickness than the anterior end, due 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. The 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. Sections are called sagittal that are 

 parallel to the plane which coincides with both chief and dorso-ventral 

 axes; frontal, those that are perpendicular to the dorso-ventral axis; 

 transverse, those that are perpendicular to the chief axis.. 



III. 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 



