3 g8 AN INTRODUCTION TO ZOOLOGY 



part in the act of nuclear fusion, the others after a while degenerating. 

 The presence of such an enormous amount of yolk renders it impos- 

 sible for the tiny mass of cytoplasm to control the whole ovum, and 

 as a result we find that the cleavages are confined to one pole of the 

 egg. Such a type of segmentation is termed meroblastic or incom- 

 plete, in opposition to the holoblastic or complete type. 



The first cleavage is indicated about three hours after 

 ovulation by the formation of a furrow in the middle of the germinal 

 disc which extends over about half its diameter. Not merely 

 does this groove fail to stretch across the disc, but it also does not 

 reach the bottom of the cytoplasm. The second cleavage occurs 

 in a plane at right angles to the first, so marking off four areas, the 

 blastomeres. Typically the third cleavage is represented by a double 

 furrow parallel with the first, so that the centre of the disc is divided 

 into two rows, each containing four cells. Hereafter the cleavages 

 become irregular, although it is sometimes possible to make out 

 a fourth fairly regular cleavage in the form of a pair of furrows parallel 

 with the second, so cutting the four cells on each side into central 

 and peripheral daughter cells. While the third and fourth cleavages 

 are making their appearance a horizontal cleavage takes place in 

 the region of the central cells cutting off from under them a central 

 unsegmented mass of cytoplasm. So that there is now a group of 

 central cells completely circumscribed and surrounding it the 

 marginal cells joining the periblast. The whole area of cells so formed 

 is termed the blastodisc or blastoderm. The horizontal cleavage 

 just noted widens until it forms a slit which is the segmentation 

 cavity or blastocoel, or as it is often termed in this form the sub- 

 germinal cavity. Above this lie the central cells and below it the 

 unsegmented cytoplasm now designated the central periblast to 

 distinguish it from the original or marginal periblast. 



Subsequent cleavages take place somewhat rapidly, until a large 

 number of cells are produced, most of which are central cells, while 

 the marginal cells are reduced to a small band around the periphery. 

 Other cleavages have also occurred in the horizontal plane, so that 

 the roof of the blastoccel is several cells thick. The nuclei of the 

 marginal cells divide without accompanying cytoplasmic divisions 

 and the daughter nuclei so produced wander out into the peripheral 

 and finally into the central periblast. So that the periblast becomes 

 transformed into a syncytium, that is, a continuous mass of proto- 

 plasm containing a number of nuclei without the corresponding 

 cell divisions. Hundreds of tiny cells are formed by this time and 

 the blastoderm begins to increase in size, partly from the growth 

 and extension of its own cells, and partly by the addition of cells 

 cut off from the periblast. The inner part of the periblast, which 



