516 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. x 



ally the greater part of its thickness consists of calcareous trabeculae 

 forming a fine sponge work. The inner surface of the shell is rough, 

 projecting into minute conical papillae, while the outer surface is 

 covered by a smooth apparently structureless layer perforated by 

 numerous fine pores. 



Segmentation. — If the egg has been fertilized it proceeds with 

 its development as it slowly travels down the oviduct. The process 

 of segmentation is accomplished during this period and consequently 

 the obtaining segmentation stages involves the sacrifice of the parent 

 hens. Owing to the difficulties in the way of obtaining a complete 

 series our knowledge remained for long fragmentary but recently (1910) 

 a number of stages have been described and figured by Patterson 

 which give a fairly complete picture of the process (Fig. 224). From 

 these data we may take it that the early phases of segmentation are 

 based on the normal plan where a meridional furrow appears travers- 

 ing, or passing close to, the centre of the germinal disc i.e. the apical 

 pole of the egg, and is followed by a second meridional furrow 

 perpendicular to the first. In the third phase there is occasionally a 

 regular set of four vertical furrows but more usually the process now 

 becomes irregular (Fig. 224, C). In the next phase also there may 

 be a fairly regular development of latitudinal furrows demarcating a 

 group of about eight cells round the apical pole but typically there 

 is no such regularity. 



The initial furrows, which make their appearance as above indi- 

 cated, gradually extend. They eat their way downwards into the 

 thickness of the germinal disc, never however cutting completely 

 through it. They also extend outwards towards the edge of 

 the disc which however again they never quite reach. The apparent 

 segments into which the germinal disc is mapped out by the 

 early furrows are therefore not really isolated from one another 

 — there being still continuity between the segments on the one 

 hand peripherally and on the other on the lower side of the disc 

 next the yolk. 



Complete blastomeres are first marked off when, about the time 

 the latitudinal furrows appear, division planes make their appearance 

 parallel to the surface, cutting off the small segments in the centre 

 from the underlying deep layer of the germinal disc. 



The later stages of segmentation are quite irregular. Division 

 planes make their appearance in all directions by which the germinal 

 disc becomes completely divided up into small segments except on 

 its lower surface and round its edge where there remains a syncytial 

 mass in which the nuclei divide without their division being followed 

 by any protoplasmic segmentation. It is to be noted that the process 

 of segmentation throughout goes on more actively towards the centre 

 of the disc, more slowly towards its margin, so that the blastoderm 

 comes to be composed of smaller cells towards the centre and larger 

 towards the periphery. 



The result of the segmentation process is that the original 



