PRINCIPLES OF ANIMAL BIOLOGY 41 



The nature of these sections has been explained in Exercise IV. While 

 still in the ovary, before growth began, the female cells were oogonia. 



26. In the uppermost row of sections which is from the inner part of 

 the uterus, find primary oocytes each containing a triangular dark body 

 with a distinct black nucleus. These triangular bodies are spermatozoa 

 which have already penetrated the primary oocytes. Some spermatozoa 

 may also be found among the oocytes. 



2c. Note the nuclei of the primary oocytes. Some will have formed 

 spindles preparatory to the first maturation division. 



2d. In the second row of sections, from a point a little lower down in 

 the uterus, observe oocytes undergoing their first maturation division. 

 The chromosomes are arranged in two quadruple bodies or tetrads. 

 Each tetrad is composed of two chromosomes brought together in a proc- 

 ess known as synapsis, the chromosomes of the pair having divided so as 

 to form four parts. The nucleus of the spermatozoon, surrounded by 

 more darkly stained protoplasm, may also be seen in some specimens. 

 Select a clear specimen, and draw. 



2e. In the third row of sections are secondary oocytes undergoing the 

 second maturation division. The secondary oocyte is surrounded by a 

 thick membrane. Within this membrane, at the surface of the oocyte, 

 is found in some sections, a small dark object, the first polar body. This 

 and the secondary oocyte constitute the two daughter cells formed by the 

 first maturation division described in 2d. If the polar body is not 

 seen, explain its absence. 



In the secondary oocyte observe the spindle, bearing two double bodies, 

 the dyads. Each dyad is half of one of the tetrads described in 2d. When 

 this second division is completed, two single bodies (chromosomes) will 

 have gone into the second polar body (a very small cell), and two remain in 

 the mature ovum. The nucleus of the spermatozoon may be visible in 

 some specimens. 



Draw a specimen showing a spindle with clear dyads. 



2f. In the fourth row of sections, the second maturation division is 

 already completed and the two polar bodies are visible at the surface of 

 the mature egg in some of the specimens. (The first polar body in some 

 instances adheres to the inner surface of the egg membrane.) 



The first polar body in some animals divides so that out of the original 

 oocyte, four cells are formed, one of which is the mature egg and the 

 others the polar bodies which are without function. 



Observe in the interior of the mature egg the two large vesicular 

 nuclei, containing scattered granules of chromatin. One of these is the 

 egg nucleus , the other the sperm nucleus. These fuse to form a cleavage 

 nucleus and this fusion constitutes the final step in fertilization. 



2g. In the fifth row of sections, the fertilized ovum is undergoing 

 division or cleavage. Two-celled and four-celled embryos will be found. 



