200 PRINCIPLES OF ANIMAL BIOLOGY 



Eggs are very often enclosed in a membrane or shell, particularly 

 among species that lay their eggs on land where evaporation must be 

 retarded. These envelopes may be of a chitinous nature, as among 

 insects, or composed of keratin which resembles chitin, or they may be 

 impregnated with calcium salts. The shell of the egg of the domestic 

 fowl is composed of three layers. The inner layer is composed of limy 

 particles with conical faces pointing inward. These particles do not 

 fit closely, and air may pass between them. Outside this layer is a com- 

 pact sheet of calcareous strands which also permits the slow passage of 

 gases. On the outer surface of the shell is a third layer, the cuticle, 

 which appears to be structureless except that it is penetrated by pores. 

 Within the shell is a membrane consisting of two layers of fibers crossing 

 one another in various directions. The envelope as a whole is calculated 

 to prevent excessive evaporation, and yet it permits the passage of gases 

 necessary for the respiration of the egg and embryo. Indeed, air begins 

 to penetrate the shell soon after the egg is laid and accumulates in a space 

 between the two layers of the membrane within the shell at the large end 

 of the egg. 



Time and Mechanism of Fertilization. — Eggs and spermatozoa are 

 brought together in fertilization by breeding behavior or some sort of 

 affinity, as described in the preceding chapters. The time of their 

 union, particularly in relation to the stage of oogenesis, is very variable. 



In Ascaris megalocephala, a roundworm parasitic in the intestine of 

 the horse, the spermatozoon enters the oocyte about the time of the 

 formation of the spindle of the first division. It remains in the oocyte 

 during the succeeding divisions. In the frog, rabbit, and some others 

 the spermatozoon enters after the first polocyte is formed but before 

 the second. In the sea urchin the spermatozoon does not enter until 

 after both divisions. 



In eggs having a shell at the time of fertilization, there is an opening 

 through which the spermatozoon enters (Fig. 171m). In naked eggs,, 

 the spermatozoon may enter anywhere. Usually only one male cell 

 penetrates an egg. Some change of a chemical or physical nature takes 

 place in the protoplasm of the egg when a spermatozoon unites with it, 

 such that no other spermatozoa can be drawn in. When by accident 

 two or more spermatozoa gain entrance at the same time, al')normalities 

 of development are likely to occur. However, in some animals numei'ous 

 spermatozoa regularly enter the egg; but the imcleus of only one of them 

 unites with the egg nucleus. 



Cleavage. — Shortly after fertilization, within a time measured by 

 minutes or hours in most animals, the fertilized egg begins to divide. 

 This division, which is repeated in rapid succession until the egg is con- 

 verted into many cells, is called cleavage or segmentation. In the follow- 



