ON THE (iASTlUL.VTIOX l\ PIlTllOM V/,<>\. 17 



;i lliii'k crowd of iiiacroiiu'ros {mac///.), :illlioiii;li lliis lias iiiucli 

 (lecrciised in tliickness as {'omparcd willi the correspoiuling pail in 

 the foregoing stage. Moreover, the inner zone of tlie layer is no 

 longer loose in structure, but is very compact. The same is 

 also true of the micromeric layer. Consequently, the demarka- 

 tion against the segmentation ca.vity is no longer an irregidar 

 line, but is even and sharj). We may therefore say that here 

 the segmentation is over and that the ovum is fullv sfrown in 

 the axial diameter l)y multiplication of the component cells. 



It should be further noticed that the floor of the segmenta- 

 tion cavity is not flat, but is excavated, so that the massive layer 

 of the floor and the thinner cupola layer pass over gradually, 

 not abruptly, into each other. The animal hemisphere is fully 

 distended ; the distension seems to be maintained by a more 

 or less viscous fluid of albuminous nature, which fills up the 

 segmentation cavity and which coagulates when hardened and is 

 affected, though faintly, by staining reagents. 



A median sagittal section through an ovum, in which the 

 boundary groove is just manifest, is represented in Fi(/ 14. 

 A change which draws attention first of all is a shallow indenta- 

 tion formed at the junction of the micromeric layer witli the 

 macromeric (b.^.) ; this is the boundary groove cut through. No 

 less conspicuous is another change met with in a small area of the 

 micromeric wall (imc. ep.), which forms the upper half of the 

 groove. Here it is thinned out, being reduced into a layer one 

 cell deep, while the remaining parts are still in the same condition 

 as before ; so that the transition of these parts is no longer gradual 

 but abrupt, the micromeric layer one cell thick passing at once into 

 the multicellular macromeric wall {Figs. 14 and 23). The micro- 

 meric layer under notice is not an aggregation of indifferent cells 



