254 COMPARATIVE ANATOMY 



taken through the middle of the ovum at right angles to 

 the plane of the paper. Fig. 59, A, is such a section of the 

 stage shown in fig. 58, A. The egg is just beginning to advance 

 beyond the blastula stage, and has a large segmentation-cavity 

 or blastocoele roofed in above by several layers of small cells, 

 the outermost layer, covering the upper hemisphere of the 

 egg, having the form of a deeply pigmented columnar 

 epithelium. The floor of the blastocoele is formed by a 

 mass of large yolk-cells occupying the whole of the lower 

 or white hemisphere of the egg. At db. the dorsal lip of 

 the blastopore is seen in section, and it appears that it is 

 formed by the tucking in of some of the small pigmented 

 cells along a short horizontal line just below the equator of 

 the egg. Fig. 59, C, corresponds with the similarly lettered 

 stage in fig. 58. It shows that the dorsal lip of the blastopore 

 has travelled through an arc of some 45° towards the lower pole 

 of the egg, and that this movement has been accompanied' 

 by a continuous infolding of the dorsal lip of the blastopore. 

 The process can best be understood by fixing one end of 

 a strip of cloth to some curved surface, and folding back 

 the free end of the strip over the attached end. If a piece 

 of chequered cloth be used and the chequers are taken to 

 represent cells, then, as one pulls down the folded edge, one. 

 can see how the chequers on the outer surface are continually 

 rolling inwards over the edge of the fold and are forming 

 the roof of a narrow space enclosed between the inner limb 

 of the fold and the curved surface. Fig. 59, C, shows that 

 the dorsal lip of the blastopore has advanced in this manner, 

 that the pigmented cells actually roll in over the advancing 

 lip of the blastopore, and that the result is the formation 

 of a double fold of tissue covering in a narrow slit-like space 

 which is the first indication of the gut or enteron. Between 

 the limbs of the fold there is a yet narrower slit-like space 

 which communicates above with the blastocoele and is evi- 

 dently a continuation of the latter. 



Fig. 59, E, shows a more advanced stage of the process. 

 The dorsal lip of the blastopore has now travelled nearly 

 as far as the lower pole of the egg, and the double fold 

 formed by its advancement is longer, and has extended 

 upwards, forming the boundary of a relatively spacious cavity 

 which is evidently an extension of the slit-like enteron of 



Digitized by Microsoft® 



