. Chordate morphology. Morphology (Animals); Chordata. 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 Nelse
. Chordate morphology. Morphology (Animals); Chordata. 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 t
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