THE ULTIMATE SELF-ELEMENT. 85 



(V) Among the loose colonies which some Protozoa form, 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. 83, 84). 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 con- 

 nected. 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 justifiable to speak with Weismann and others of 

 the "immortality of the Protozoa." In a certain sense, too, as we 

 shall see, it is justifiable to speak of the immortality of the reproduc- 

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

 cells escape, before its death, to live on, as new organisms, inclosing 

 new sets of reproductive cells. Again, there is similarity between the 

 Protozoa and the reproductive cell. 



But in some of the loose colonies (for example, Volvox) we see 

 the beginning of the change which introduced death as a constant 

 phenomenon (see fig. p. 122). 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 constant in higher animals. The 

 only marked differences are — (a) that the body of the metazoon is 

 more than a loose colony of cells ; (3) 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. 



VI. General Origin 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 

 stratum is now frequently termed a mesoglcca. 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 mesoglseal cells ; 

 the primitive male-cells, which divide into numerous minute spermatozoa, are 

 the reverse. 



In coelenterates the phenomena are of much interest; the origin of the sex- 

 cells is very diverse. Some time ago considerable emphasis was laid by E. van 



