64 REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



DIAGRAM III. 



The Reproductive Cells, Maturation, Fertilisation, 

 AND Segmentation. 



At the foot of the page, the maturation of the ovum is shown in A, 

 B, C. In B, the first polar globule is being formed ; in C, the second. 



In the line above, (l) is a niother-sperm-cell, comparable to an ovum. 

 In (2), it has segmented into a ball of cells or spermatocytes, which in 

 (3) are becoming differentiated into spermatozoa. 



At D, a spermatozoon is represented entering the ovum ; in E, the 

 sperm-nucleus and the reduced nucleus of the ovum are about to com- 

 bine ; in F, the segmentation-nucleus is formed. 



In the upper part of the page, four kinds of segmentation are shown. 

 In A and B, the segmentation is holoblastic ; in C and D, it is mero- 

 blastic. 



A I is an ovum with diffuse yolk ; A 2 is the external aspect of the 

 blastosphere ; A 3 is a section of the blastosphere ; A 4 is a section of 

 the invaginated blastosphere or gastrula. 



B I is an Amphibian ovum with the yolk towards the lower pole ; B 2 

 is the blastosphere which results from unequal segmentation ; B 3 is a 

 section showing the segmentation-cavity; B 4 shows the beginning of 

 the gastrula. 



C I is an Arthropod ovum with central yolk ; C 2 shows the peri- 

 pheral or superficial segmentation ; C 3 shows the pyramids into which 

 the yolk becomes divided ; C 4 shows the small invagination which 

 represents the gastrula. 



D I is an ovum with much yolk, as in Birds ; D 2 shows the discoidal 

 segmentation ; D 3 shows a more advanced blastoderm ; D 4 gives a 

 slight hint of the primitive streak which appears on the embryonic area 

 of Amniota. It corresponds to the gastrula stage. 



It will be noted that the figures in each horizontal line, I, 2, 3, 4, 

 represent approximately similar stages. The yolk is shaded throughout. 



