ON IHK (iASTRl I,ATU)N I\ I'lTROjM V/.oN. 41 



tlio conical otuiiiciicc (c.rm.) to tli.^ Ixxindary groove {/>.(/.) is 

 very well seen. Tlie inieroineric part is transformed into a 

 eolniiiiiai- epitlieliiini (niic.ep.) at a small area in tin; groove. 

 X CI. 



Fi(/. IT) — A median sagittal sectit)n through an ovum, alioiit the same in 

 dovelojiment as tlie snrfaco view represented in Fif/. 4a. Tiie 

 conical eminence (c.cin.) is distinct ; accordingly, the Ixaindary 

 groove (h.tj.) is deep, and a small lield of the macromeric hemi- 

 s|)h(a'e innnediately below the emintnice is tlattened (4..). x CA. 



Fitj. 1() — A median sagittal section through an ovum of about the same 

 stage as that shown in Fig. 5«. The commencing gastrula 

 invagination {hp.) is well seen. The conical ennnence and the 

 boundary groove are conspicuous. The floor of the segmentation 

 cavity opposite the invaginating blastopore {hp.) is a little raised 

 into that cavity, x G4. 



Fig. 17 — A median sagittal section througli an ovum a little older than 

 the surface view given in Fig. da. The boundary groove has 

 somewhat shifted anteriorly, being ])ressed by the invagination of 

 the macromeres which formerly formed the conical eminence and 

 which now take an epithelial arrangement {mac.cp.). The arch- 

 enteron has appeared for the first time. The differentiation of 

 the micromeres into the columnar epithelium {mic.ep.) has ex- 

 tended over an increased area. The wrinkled chorion {di.) is to 

 be seen, x 64. 



/,7,y. 18 — A median sagittal section through an ovum of about the same 

 stage as the snrface view shown in Fig. la. The archeuteron 

 {ar.en.) and tlie macromeric epithelium {mac.cp.) roofing the cavity 

 have increased in extent. The boundary groove {h.g.) has shifted 

 ftirther anteriorly, and the segmentation cavity (.s.c.) is nnich 

 reduced. Some macromeres outside the anterior blintl end of the 

 archenteron are in a disturbed state {v.-^). x 80. 



l,';,j^ \\) — A median sagittal section through an ovum as much advanced as 

 the stage represented in Fig. 8a. Not only the archenteron 

 {ar.en.) and the macromeric roof of it [mac.ep.) have increased 

 in extent, but in the posterior section of the roof there 

 have come into view the raicromeric epithelium {i.mic.ep.), 

 which may })e distinguished from tlie macromeric [lart lying 

 anteriorly tn the point marked with -i- by the regular arrangement 

 of cells as well as bv tlieir smaller nuclei. The boundary groove 



