vm] CENTRO-LECITHAL SEGMENTATION 121 



connection with differentiation of parts, the actual pattern 

 or arrangement of the blastomeres in a segmenting egg is a 

 matter of interest. It appears that this follows the law of 

 the minimal area of surface contact, according to which any 

 drops of liquid are compelled by surface-tension to arrange 

 themselves so that the sum of their surfaces is reduced to 

 a minimum. Where this law appears to be broken, the 

 cause is probably to be sought in the different consistency 

 of different parts of the egg 1 . 



All the examples of segmentation mentioned hitherto have 

 been drawn either from eggs having total cleavage, or from 

 telo-lecithal eggs in which a polar blastoderm is formed. 

 Especially among the Arthropods a different type of cleav- 

 age is found, known as centro-lecithal. In these eggs the 

 yolk forms a central mass and is enclosed in a sheath of 

 yolk-free protoplasm which extends over the whole surface 

 of the egg, though it is usually thicker in one region than 

 elsewhere. In eggs of this type the conjugation of the egg 

 and sperm nuclei usually takes place deep in the yolk and 

 the division of tr^e zygote nucleus is not followed by any 

 immediate cleavage of the egg protoplasm. The zygote 

 nucleus divides, its two daughter nuclei separate and divide 

 again, and the process is repeated until a number of nuclei 

 are scattered through the yolk. Then some or all of them 

 migrate to the surface and enter the superficial layer of 

 protoplasm, which thus becomes a blastodermic epithelium 

 in which the cells are more or less clearly marked. In these 

 eggs there appears to be no necessary relation between the 

 position of the first division-figure and the orientation of 

 the embryo. The position of the various parts of the embryo 

 is determined by a definite polarity of the egg itself, and the 

 subsequent fate of each of the numerous segmentation 

 1 Cf. PRZIBRAM (1908). 



