738 LIFE : OUTLINES OF GENERAL BIOLOGY 



the lining of the body-cavity and the enswathing of the food-canal ; 

 the kidneys; the blood, the heart, and the blood-vessels. In the 

 case of outgrowths from the embryonic gut, the outside of each 

 pocket must be the mesodermic enswathing, so that all these pouch- 

 ings are in part endodermic and in part mesodermic. Similarly, 

 the mesodermic backbone, if there is one, develops around the 

 endodermic notochord, which it replaces in whole or in part. Con- 

 nective tissues, vascular system, and unstriped muscles are formed 

 from mesenchyme cells (see above). In triploblastic animals the 

 essential reproductive organs or gonads are associated with the 

 mesoderm, the ovaries or testes developing on the waU of the body- 

 cavity. But it is clearer to keep the origin of the gonads apart 

 from that of the other organs or tissues. They arise from an early 

 segregation of primitive germ-cells, which take no part in body- 

 making . 



GENERALISATIONS.— (i) The ovum-theory or ceU-theory is a 

 biological commonplace, though it should also be a problem; yet it 

 is an unfamiliar fact to the ordinary citizen that every multi- 

 cellular animal, if reproduced in the ordinary way, starts in life as 

 a single cell — the fertilised ovum. The Metazoa begin where the 

 Protozoa leave off — as single cells. Fertilisation does not make the 

 egg-cell double; there is merely a more complex and more vitalised 

 nucleus than before. All development takes place by the division 

 of the fertilised egg-cell and of its descendant cells. Omnis cellula e 

 cellula; and omne ovum ex ovo. 



(2) Gastrsea Theory. As we have already noticed, there is in 

 many types a more or less clearly marked gastrula stage^ — a two- 

 layered sac of cells. This led Haeckel to suggest that the ancestor 

 of many-celled animals was gastrula-like ; and he called the hypo- 

 thetical primitive form the gastrcea. He regarded the ontogenetic 

 gastrula as the individual's recapitulation of the phylogenetic 

 gastraea. There is nothing far-fetched in this hypothesis, for the 

 Metazoa must have begun somehow, and some very simple forms 

 like Protohydra and even certain Turbellarian worms are not far 

 removed from elongated gastrulae. It is not inconsistent with 

 Haeckel's theory to suppose that there may have been other 

 primitive forms of Metazoa, such as a hollow ball of cells like the 

 common Vol vox of to-day. But a diploblastic thimble-hke sac with 

 a mouth is nearer the simplest Metazoa now living, and on general 

 grounds it must be regarded as a promising presumptive ancestor. 

 The speculation must be supplemented, however, by an inquiry 

 into the actual conditions operative to-day and in the distant past, 

 that may account for the invagination of a blastula to form a 

 gastrula. One of these conditions may be found in the more rapid 

 division of the cells of the upper, less yolk-laden hemisphere of the ball. 



