Embryology and the position of the anus

In early development each animal starts as a single cell. Soon this cell begins to divide, first into two cells, then four, then eight, sixteen, and so on (Figure 1.5(a-c)).

Eventually a hollow ball of cells is produced, called the blastula stage (Figure 1.5(d)). A pocket of cells then moves inwards, forming the precursor of the gut and other internal structures. The opening of this deep pocket is called the blastopore.You can imagine pushing in the walls of a hollow rubber squash ball with your thumb to produce a model of this embryonic pattern, known as the gastrula stage (Figure 1.5(e-g)).

Embryologists noticed some time ago that animals fall into two large groups depending on the relative orientation of the mouth and anus. The classic story is that in most invertebrates (the protostomes), the blastopore becomes the mouth (Figure 1.5(h)), whereas in others (the deuterostomes), including the chordates, this opening becomes the anus (Figure 1.5(i)), and the

Fig. 1.5 Embryonic development: (a-g) sequence of cell division in amphioxus, from the single-cell stage (a), through the blastula stage (d),to the gastrula stage (g). (h) Fate ofthe blastophore in protostomes,and (i) in deuterostomes. [Figures (a-g), after Hildebrand and Goslow, 2001, copyright © 2001 John Wiley & Sons, New York; (h,i),after Jefferies, 1986.]

Fig. 1.5 Embryonic development: (a-g) sequence of cell division in amphioxus, from the single-cell stage (a), through the blastula stage (d),to the gastrula stage (g). (h) Fate ofthe blastophore in protostomes,and (i) in deuterostomes. [Figures (a-g), after Hildebrand and Goslow, 2001, copyright © 2001 John Wiley & Sons, New York; (h,i),after Jefferies, 1986.]

mouth is a secondary perforation. Such a dramatic turnaround, a switch from mouth to anus, seems incredible. Note, however, that many protostomes show deuterostomy, and this condition may be primitive and shared by all Bilateria (Eernisse and Peterson, in press). Nevertheless, this peculiarity of embryological development appears to solve the question of the broader relationships of chordates.

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