12 WALLACE. 



opaque rim is noticeable, while in preparations with osmic acid, 

 the rim becomes much darker than the rest of the blastoderm. 

 By the study of surface mounts and from sections, this thick- 

 ness was found to be due to the greater thickness of the peri- 

 blast in that region, and also to the accumulation of huge 

 periblastic nuclei. The presence of oil globules increases the 

 effect, especially in the living egg. 



PI. II, Fig. lo, is a reproduction of Dr. Clapp's Fig. i, d. 

 She says: "In Fig. i, d, this notch is seen at a little dis- 

 tance behind the embryo ; a shadowy connection may be traced 

 between the germ-ring and the embryo." While at this time 

 only the appearance of a germ-ring exists, the " shadowy con- 

 nection " between the "germ-ring" and the embryo has a 

 more substantial basis. The same stage is given in PI. II, 

 Fig. 5. A longitudinal section through the embryo and the 

 margin of the blastopore is shown in PI. Ill, Fig. S. A few 

 cells, marked "m," are seen lying beneath the ectoderm and 

 reaching from the posterior end of the embryo almost to the 

 lip of the blastopore. Sections all around the blastopore show 

 no thickening except of periblast. 



Summary. — In the egg of Batrachus there is a centripetal 

 growth of cells at the embryonic pole, the ingrowth having a 

 voluted outline in sections. Around the remainder of the 

 blastoderm there is not even the appearance of an invagination, 

 but only a slight thickening due to an ingrowth of cells from 

 the ectoderm, and a few loose cells which may represent a true 

 germ-ring found as a layer in ordinary forms. The peripheral 

 thickening gradually fades out, first at the anterior pole, until 

 the last remnant is found in a few cells lying beneath the ecto- 

 derm, forming a linear streak from the posterior end of the 

 embryo to the lip of the closing blastopore. In the gradual 

 disappearance of the thickening, beginning at the anterior pole 

 and continuing on either side toward the posterior pole, accom- 

 panied by the lengthening of the embryo, we see a highly 

 modified form of concrescence. 



Woods Holl, Mass., 



September, 1896. 



