90 



THE EVOLUTION OF SEX. 



and which bridge the gulf between the unicellular animals and 

 the ^Metazoa, there is seen the beginning not only of the formation 

 of a "body," but also the setting apart of special reproductive cells 

 (see figs, on pp. 88, 89). On this point more emphasis must be 

 laid. The ordinary Protozoon is a single cell, and forms no body. 

 It divides indeed, and multiplies accordingly, but the products 

 of division go asunder, whereas in the segmentation of the ovum 

 they remain connected. In most Protozoa, there is continual 

 self-recuperation ; in most, division occurs without any loss ; in 

 most, there is no distinction between parent and offspring ; in 

 most, as there is no body, there is no death. Thus it is that, 

 with one weighty caution to be afterwards noted, it seems justi- 

 fiable to speak with Weisraann and others of the "immortahty of 

 the Protozoa." In a certain sense too, as we shall see, it is 

 justifiable to speak of the immortality of the reproductive cells 

 in higher animals. The body dies, but the reproductive cells 

 escape, before its death, to live on, as new organisms, enclosing 

 new sets of reproductive cells. Again there is similarity between 

 the Protozoa and the reproductive cells. 



But in some of the loose colonies {e.g., Volvox), we see the 

 beginning of the change which introduced death as a constant 

 phenomenon (see fig. p. 130). The cell, which starts one of 

 these colonies, divides; the products of division, instead of going 

 apart as usual, remain connected : a loose body of many cells 

 is thus formed. In this cluster of cells, certain elements are in 

 turn set apart and eventually adrift, as reproductive cells. They 

 start new colonies, and thus we are introduced to what is con- 

 stant in higher animals. The only marked differences are — {a) 

 that the body of the Metazoon is more than a loose colony of 

 cells ; {b) that the reproductive elements are usually liberated 

 from some definite region or organ ; and {c) that they are more 

 markedly differentiated as male and female cells. 



§ 6. Ge7ieral Origi?i of the Sex-Cells. — Except in the lowest 

 invertebrates, the sponges and coelenterates, the reproductive 

 elements almost always arise in connection with the middle 

 layer (mesoderm or mesoblast) of the body. 



Neither in sponges nor in coelenterates is there a middle layer exactly 

 comparable to the mesoderm of higher animals ; the less definite middle 

 stratmn is now frequently termed a mesoglsea. In sponges, we already 

 mentioned that the reproductive cells simply arise here and there among 

 the other elements of the stratum. The ova are highly nourished mesoglaeal 

 cells ; the primitive male cells, which divide into numerous minute sperma- 

 tozoa, are the reverse. 



