first polor body f 99 nucleus 



vitelline membrane 



ISOLECITHAL 



B 



polar bod/ 



vitelline membrane 



jelly layers 



MESOLECITHAL 



MESOLECITHAL 



btastodisc 



outer shell membrane, 

 inner shell membrane 



nucleus of Pander 



neck of lotebra 

 latebra 



white yolk layer 

 between layers 

 of yellow yolk 



chaloza 



polar body 



light albumin 

 dense albumin 



yolk globule 



zona pellucida 



egg nucleus 



albumin 

 shell membrane 



D TELOLECITHAL 



ISOLECITHAL 



Figure 7-1. The general form of the chordate egg. A, the isolecithal egg of omphioxus; B, meso- 

 lecithol, shelled egg of the lamprey; C, mesolecithal, jelly-coated egg of frog; D, extreme telolecithal 

 egg of the chick with accessory materials and shell; E, isolecithal egg of the opossum which retains 

 something of the structure of the shelled egg of the monotreme. (A, C after Huettner, 1949; B after 

 Nelsen, 1953) 



misleading in that the cells at the vegetal pole are always 

 larger than those at the animal pole; the difference then is a 

 matter of judgment. When there is a great discrepancy in 

 the size of the resulting cells, one speaks of macromeres 

 versus micromeres. 



When the egg is of a teleoiecithal type, the cleavage is de- 

 scribed as meroblastic (Figure 7-9). The terms discoidal and 

 superficial describe the extremes of this division type. The 

 first refers to extreme telolecithal eggs with a cytoplasmic or 

 germinal disc on the top to which the cleavages are re- 

 stricted. Superficial refers to a less yolky egg in which the 

 cleavages, at first only surface events, gradually extend 

 through to the vegetal pole (Figure 7-12). The discoidal 

 type is observed in the hen or reptile egg; the superficial type 

 occurs in some fishes. 



CLEAVAGE, GASTRULATION, ORGANOGENESIS 



For a better understanding of the early stages of devel- 

 opment of chordates, some of the details of several types can 

 be compared. 



Amphioxus The ovum of Amphioxus is small and isoleci- 

 thal or hololecithal (Figure 7-1 A). The nucleus and its en- 

 closing ooplasm is displaced toward the animal pole. This 

 results in the first meiotic division of the nucleus, with a 

 polar body lying at the animal pole. The egg is covered by 

 a thin vitelline membrane, and outside of this lies the polar 

 body (polocyte). 



The eggs are shed into the water at a time when the 

 males are shedding sperm. Fertilization occurs by pene- 

 tration near the vegetal pole by a single sperm cell. As a re- 

 sult of this penetration, a perivitelline membrane forms be- 

 neath the vitelline membrane, and, as a result of loss of 

 water by the ovum, these two membranes, now fused, lift 

 off the egg surface to form a perivitelline space (Figure 7-2). 

 These two membranes combined are identified as the ferti- 

 lization membrane. 



The male pronucleus (from the sperm) moves upward 

 through the egg and meets the female pronucleus moving 

 downward. The two fuse (or lose their separate identities) 

 above the center of the egg and set up the first division 

 spindle. Cleavage is holoblastic; the first cleavage plane se- 



194 



EMBRYOGENESIS OF THE CHORDATES 



