Embryonic Development; Differentiation of Tissues - 273 



AXIAL FILAMENT SHEATH OF TAIL CENTRIOLE ACROSOME 



MITOCHONDRIA^ \ NUCLEUS 



TAIL 



MIDDLE 

 SECTION 



HEAD 



Fig. 15-3. Typically the sperm, or male gamete, is a highly specialized, exceedingly motile, haploid 

 cell. 



time, the locus of polar body formation con- 

 stitutes an important determinant of future 

 development. The egg is a polarized cell. One 

 pole, designated as the animal pole, is 

 marked by the point of appearance of the 

 polar bodies, while the other, called the 

 vegetal pole, lies at dead center in the oppo- 

 site hemisphere of the egg (see p. 277). Thus 

 an anterior-posterior axis of development is 

 provided along the line that extends from 

 pole to pole — even before the egg cell has 

 been fertilized. 



The outermost membrane of the unfertil- 

 ized egg, a specialized sort of pellicle, is 

 called the vitelline membrane (Fig. 15-1). 

 Upon activation (p. 274) this membrane be- 

 comes lifted away from the cytoplasmic sur- 

 face of the egg, giving rise to what now is 

 called the fertilization membrane (Fig. 15-2). 



The Sperm Cell. Typically the sperm is an 

 extremely small, highly motile, haploid cell. 

 It consists of three differentiated parts, 

 namely, head, middle piece, and tail (Fig. 

 15-3). 



Perhaps the most essential feature of the 

 head is the highly condensed haploid nu- 

 cleus. Each of the many primary spermato- 

 cytes produced in a testis gives rise to four 

 sperm, as a result of the meiotic divisions. A 

 haploid set of chromosomes, brought into the 

 egg by a sperm, participates in the formation 

 of the zygote nucleus (Fig. 15-4), and this con- 

 stitutes the essence of fertilization. 



At the tip of the head of a sperm, at least 

 in many cases, is a highly specialized struc- 

 ture, called the acrosome (Fig. 15-3). For 

 some years it was thought that the acrosome 

 facilitates penetration of the sperm into the 

 egg, but evidence was lacking until recently. 

 Now, however, the work of Dan and Wada 

 in Japan and Colwin and Cohvin in the 

 United States has demonstrated that the 

 acrosome has two functions. First, it dis- 

 charges an exceedingly fine thread, the acro- 

 some filament. This maintains a connection 

 between the sperm head and the surface 

 of the egg. Then an amoeboid protrusion 

 from the egg surface, the fertilization cone, 



Fig. 15-4. Fertilization and cleavage of the egg of a roundworm (Ascan's). A, fertilization proper: the egg and 

 the sperm nuclei are fusing to form the diploid zygote nucleus, which (in this case) bears two chromosomes from 

 the female parent and two from the male parent. B, early anaphase, first cleavage. C, telophase, first cleavage. 

 D, polar view of the metaphase of the first division, showing the two pairs of chromosomes of the zygote. (Photo 

 D, slightly retouched. Copyright, General Biological Supply House, Inc.) 



