7 



Embryogenesis of the 



Chordates 



Embryology contributes much to an understanding of 

 adult anatomy and therefore to comparative morphology. 

 In the preceding chapters, the developmental aspects of 

 the skeletal system were reviewed; in each of the following, 

 the embryological contributions will be described. Embryo- 

 genesis, the early stages of development before highly differ- 

 entiated tissues have been achieved, should be described 

 separately for two reasons; to assess its contribution to the 

 history of the vertebrates and to serve as a reference base 

 for the accounts of later events. 



THE EGG 



It will not be necessary to review the production of sperm 

 cells and ova, for the details of meiosis and gametogenesis 

 (maturation) are fully described in texts of histology, cytol- 

 ogy, and genetics. 



The ovum is considered a cell, but differs from most cells 

 in its developmental potential and in its relatively large 

 size and contained yolk. In those with little yolk, the nuc- 

 leus is central and the yolk is evenly scattered in the ooplasm 

 or the cytoplasm of the egg cell. As the amount of yolk is in- 

 creased, it tends to sink down, shifting the clear area of 

 ooplasm enclosing the nucleus upward. During maturation 

 the nucleus is near the surface and by its divisions two 

 polar bodies are produced. The first of these polar bodies 

 may divide into two. In eggs with a great deal of yolk, the 

 nucleus lies in a dorsal patch of ooplasm. A top and under- 

 side to the egg cell are indicated either in terms of the posi- 

 tion of the polar bodies (polocytes — Figure 7-1) or by the 

 separation of yolk and ooplasm. At the top is the animal 

 pole, at the bottom the vegetal pole. 



The surface of the ovum is generally covered by a thin 

 membrane described as the vitelline membrane. Outside of 

 this there is a chorion (zona pellucida), secreted by the follic- 

 ular cells, which nourish and help produce the ovum (the 

 corona radiata of the follicular ovum). Outside of the chor- 

 ion may be layers of jelly, as in the frog, or a zona radiata. 

 These are actually a part of the chorion. There are many 

 different viewpoints about the nature of the membranes 



enclosing the ovum and the terminology used to describe 

 them. 



The amount of yolk is important in determining the kind 

 and path of development (Figure 7-1). Isolecithal or holo- 

 lecithal eggs are those in which the yolk is small in quantity, 

 fine grained, and evenly scattered throughout the egg — ex- 

 cept for the area surrounding the nucleus. In some, yolk is 

 more concentrated centrally (centrolecithal). Isolecithal 

 eggs are small in size and divide readily into cells when 

 development begins. Since the yolk is not prominent, they 

 might be described as alecithal. Eggs with more yolk, dis- 

 tinctly concentrated at the vegetal end, are described by 

 the term mesolecithal. Eggs with a great deal of yolk and 

 with a thin germinal disc on top are described as teloleci- 

 thal. Sometimes telolecithal is used for any degree of vegetal 

 yolk concentration. 



An egg with little yolk must either receive nutrition from 

 the maternal animal, as in the case of the human, or it can 

 produce a very small larva which must soon begin to feed 

 in order to grow. An egg with a larger amount of yolk 

 can develop to a later stage or a more complete stage; the 

 larva is larger and better prepared to fend for itself when 

 its yolk supply is exhausted. In the case of the chicken or 

 the reptile, with a very large telolecithal egg, the young are 

 hatched nearly fully differentiated and fairly large in size. 

 In the case of the bird, with some parental care, they quickly 

 become independent. 



The amount of yolk also determines the type of cleavage 

 or division. An egg with little yolk cleaves rapidly and com- 

 pletely. As the amount of yolk increases, the difficulty of 

 cleavage increases, and in a large telolecithal egg part of 

 the yolk mass never becomes intracellular. 



Since there is every gradation between the essentially 

 alecithal egg and the teleolecithal egg, it is difficult to apply 

 these terms and it is not uncommon to have different terms 

 applied to the same egg. Since the type of cleavage is tied 

 to the type of egg, the terms of cleavage are also difficult to 

 define, and perhaps, are redundant. 



Complete and rapid division of the egg is described by 

 the term holoblastic (Figure 7-2). The resulting cells of 

 .several cleavages can be equal to each other in size or un- 

 equal. This dichotomy of equal versus unequal is rather 



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