^^^J ZOOGENESIS '^^"^'^ 



any multicellular animal may be regarded as represented by a 

 vast number of fractions grouped in various ways which, when 

 added together, equal the number one, and which were all derived 

 from an original number one. 



Coming back to the single cell, there is no reason whatever for 

 assuming that complete separation of dividing cells is a more 

 primitive condition than adhesion of cells after division, or pre- 

 ceded adhesion. In fact, the great rarity of complete separation 

 of cells after division in the animal world taken as a whole almost 

 suggests that adhesion, not separation, is the primitive condition. 

 Therefore the statement commonly made that the single celled 

 animals or protozoans are the most primitive of the animals, and 

 preceded in appearance the multicellular types, has nothing to 

 support it. The only logical assumption is that the appearance of 

 unicellular and multicellular animal types was simultaneous — 

 perhaps even that the latter appeared first (fig. A, p. 138). 



Cells which after division remain in contact may adhere irreg- 

 ularly, resulting in the formation of a more or less unorganized 

 mass. Essentially such a condition is characteristic of the great 

 group of sponges, in which creatures many of the constituent cells 

 are almost wholly independent of each other and suggest masses 

 of protozoans packed closely together. 



Cells which after division remain in contact may adhere regu- 

 larly, resulting in the appearance of a series of geometrical forms 

 (figs. iio-ii7,p. 185). Regular division of cells followed by regu- 

 lar adhesion leads to the formation of a hollow ball of cells called 

 a blastula (figs. 113, 114, p. 185). The blastula collapses, like a 

 rubber ball with one side pushed in, into a cup with an outer and 

 an inner layer of cells called a gastrula (figs. 116, 117, p. 185). The 

 typical gastrula has an axis passing through the center of the 

 opening and of the opposite pole, and the radii about this axis are 

 everywhere the same — in other words the typical gastrula is radi- 

 ally symmetrical about its only axis. 



If the radially symmetrical two-layered gastrula (figs. 116, 117, 

 p. 185) should become adult, there would result a radially symmet- 

 rical animal composed of two layers of cells, which would be of 

 quite the same nature as a hydra or a sea-anemone. The whole 

 group of the Coelenterata — hydras, corals, sea-anemones, sea- 

 pens, hydroids, alcyonarians, gorgonians, antipatharians, jelly- 

 fishes, and numerous other types — represent animals which pos- 



[2-39] 



