40 AKT. n. — s. H ATTA. 



Fifj. 9c — Basal view of tlie same ovum. The blastoporic depression {hp.) 

 has becoine deeper and narrower, hut ventral ly it passes over 

 gradually into the vegetative field which is still spherical in shape, 

 wanting as yet the ventral blastoporic lip. 



Fù/. lOf/ — »Side view of a further advanced ovum. There is no longer seen 

 any trace of the translucent vesicle. The segmentation cavity is 

 thoroughly obliterated. The ovum now assumes a jiear-shape, 

 being larger in the anterior part and lessening in bulk post- 

 eriorly. 



Fig. \0h — Dorsal view of the same ovum. The dorsal surface is no longer 

 plain as before, but there is expressed an oval elevation (m.p.), 

 the medullary plate, along the median line of which is seen the 

 " Rückenrinne " running lengthwise. This is deeper towards the 

 posterior end and communicates with the blastopore, while ant- 

 eriorly it is shallower and fades away at length. 



/Vf/, lia — Side view of a much advanced ovum. The medullary plate is 

 converted into a pair of prominent medullary ridges (m.o\). The 

 ventral lip of the blastopore {vl.hp.) is now established. 



FI If. \\h — The .same ovum seen from the dorsal and posterior sides. The 

 blastopore shows its definitive circular form by the establishment 

 of the ventral lip {vl.hp.) 



Plate II. 



Fig. 12 — A section through both the animal and vegetative poles of an 

 ovum a little younger than that represented in Fig. 1. The 

 segmentation of the blastomeres are being actively carried on . 

 The macro- and micro-meres project into the semilunar segmenta- 

 tion cavity, and some of them are even detached and nearly free 

 in that cavity, x G4. 



Fig. 1.3 — A section through both the jwles of an ovum, the same' in 

 development as the surface vi(3sv shown in Fig. 2. The ovum is 

 ellipsoidal in shape ; accordingly, the segmentation cavity is no 

 longer semilunar in outline, but is circular. Segmentation is 

 over. The blastonierie wall {micrm. and macrm.) shows not only 

 a sharp contour against the segmentation cavity, but is reduced 

 in tliickness. x f54. 



Fig. 14 — A median sagittal section throiigh an ovum, nearly the same in 

 development as the stage shown in Fig. .'3a. The relation of 



