215 



formation of the dorsal lip. 

 pigment spots (intercellular 

 and ectoderm layers; the 

 spots become more nume- 

 rous and fuse, forming a 

 jagged line (furrow), which 

 gradually becomes smooth 

 and even. The ectoderm 

 cells adjacent to the just 

 formed ventral lip, measure 

 for ar the most part 40 fi 

 in diameter. 



The interpretation of 

 the observations on com- 

 pressed eggs, just described, 

 is aided by a study of sec- 

 tions ^) through eggs that 

 have developed under the 

 normal conditions. In sec- 

 tions the first recognizable 

 stage in the development 

 of the blastopore, is a 

 shallow furrow, which marks 

 out the dorsal lip. If this 

 earliest stage of the blasto- 

 pore be carefully examined, 

 certain points may be ob- 

 served, which make it pos- 

 sible to correlate the surface 

 and sectional observations. 

 In the first place the cells 

 lining the furrow are in 

 general smaller on the upper 

 side than on the lower side 

 (Fig. 8, a, b, c). The differ- 



The ectoderm cells line up ; isolated 

 depressions) appear between the yolk 



Fi^. 6. Median longitudinal section of Choro- 

 philus gastrula. Camera, Zeiss A 2. d. I. dorsal 

 lip. arch, archenteron. s. c, segmentation cavity. 

 V. I. ventral lip. a — b "tongue" of uncleft yolli 

 lying in front of archenteron. x space artificially 

 produced by separation of the layers. 



Fig. 7. Median longitudinal section of Choro- 



philus gastrula. Later stage than that shown in 



Fig, 6. Camera, Zeiss A 2. Reference letters as 

 for Fig. 6. 



1) Strong formalin as a fixing agent, with a very weak haemato- 

 xylin stain, has given by far the best results. The pigmentation, so 

 conspicuous in the living cells, is well retained in the sections. If the 

 eggs are kept too long in the formalin — months — they become 

 greenish. The solution finally used was 10 '^/^ formaldehyde or 25 7o 

 of the commercial formalose. 



