372 Fertilization and Early Development of Pigeon's Egg 



two-, four- and eight-cell stages. In the later stages obtained the ac- 

 cessory cleavage is not shown in the figures. This accessory cleavage is 

 then set up after the second mitotic division of the sperm nuclei. They 

 are found at the time of this division surrounded by wide cytoplasmic 

 areas free from yolk granules. The resting nuclei after the first division 

 are accompanied by small areas of a " sphere substance " which entirely 

 resembles this material. This sphere substance rather than being re- 

 garded as an organ accompanying the nucleus would seem to be an accum- 

 ulation of the products of the nuclear activity. During the migration 

 of the nuclei, the amount accumulated next to a nucleus appears small 

 (Fig. 36), but after they are settled down the substance soon gathers 

 in large quantities. Of course this statement is not meant to imply 

 that the sphere substance may not at certain times take on a definite 

 form, like a permanent organ, as in the young ova, for instance. Accord- 

 ing to the view of Van Bambeke, 97, the sphere substance is the center 

 of formation of plastic and of nutritive elements. 



To explain the peripheral migration of the sperm nuclei, Eiickert has 

 developed a theory of mutual repulsion which applies to all nuclei of a 

 like character, and is exerted by and through the means of the sphere 

 substance. The sperm nuclei show a mutual repulsion for each other, 

 which prevents their conjugation with one another. The cleavage nuclei 

 have a superior power of repulsion, and so drive the sperm nuclei from 

 the cleavage area into the yolk. The egg nucleus having no centrosome 

 and sphere, or only a slightly developed one, is on the contrary attracted 

 to the male pronucleus. The early and rapid migration of the sperm 

 nuclei out of the cleavage area is, however, a fact which does not seem 

 to fall within this explanation. The sperm nuclei for some reason mi- 

 grate to the periphery and give rise to an accessory cleavage there, while 

 the egg is still in the two-cell stage. Only a few straggling nuclei are 

 at this time remaining within the inner area of the disc. There seems 

 to be a tendency on the part of the sperm nuclei to migrate, only one 

 of them being caught at the early stage by the attraction of the female 

 nucleus; and this conclusion is certainly not inconsistent with the 

 motility of the sperms during the stage of their free existence. As an 

 active cause for the migration of the sperm nuclei, it might be assumed 

 that the activity is but the continued expression of the labile nature of 

 the protoplasm which gives the sperm its motile character during the 

 period of its independent existence. An indication of the rapid move- 

 ment of the sperm nuclei has already been pointed out, namely, that 

 the accumulation of altered protoplasm or " sphere substance " about 

 the nuclei while migrating in the germinal disc is very small. As soon. 



