Early Development of the Hen's Egg. 119 



disappear at an early cleavage stage, but the extra sperm nuclei also 

 disappear early. The writer has never found sperm nuclei after the 

 thirty-two-celled stage. __ 



XL SixTY-FOUK Cells in Surface View. — Eive and One-half 



Hours. 



This stage brings out more clearly than any we have so far con- 

 sidered the method by which the region of central cells increases. 

 At the inner ends of many of the marginal cells are large central 

 ones, which have been just recently cut off; and located centrally 

 to these are smaller cells (Fig. 25). The number of central cells 

 increases, therefore, in two ways: First, the central region grows 

 at the expense of the marginal cells, and in this manner the central 

 region gradually extends peripherally ; second, the central cells thus 

 formed multiply inter se. If we compare this stage with the pre- 

 ceding one (Fig. 23), in which there were seventeen cells in each 

 region, it will be seen at once that while the number of marginal cells 

 has increased but six, the central ones have increased twenty-four 

 (in surface view). This comparison becomes all the more striking 

 when it is stated that the central region averages two cells in depth, 

 due to the formation of horizontal cle'fts, so that there are probably 

 . a total of some eighty central cells, or an increase of sixty-three. 

 The central cells multiply, therefore, more than ten times as rapidly 

 as the marginal ones. 



The manner in which the central region becomes more than a 

 single layer deep is made clear in a study of a section of a blasto- 

 derm in this same stage of development (Fig. 26). The segmenta- 

 tion cavity is very distinct and above it the original one-layered disc 

 (see Fig. 22) is now two cells deep; and this condition has been 

 brought about not by the addition of cells from that portion of the 

 disc lying beneath the cleavage cavity, as supposed by Duval, but 

 entirely by the formation of horizontal clefts in the cells lying above 

 the cavity. As we might have expected, the method of increase in 

 cell layers is identical with that of the same process in the pigeon 

 blastoderm. At this stage there is no possibility of cells being added 

 to the disc from the central periblast, because that region is entirely 

 void of nuclei. 



