Mr. W. S. Kent on the Embryology of Sponges. 141 



the same view, are regarded by me as tlie equivalent, not of a 

 single body or person, but as a special aggregation of innume- 

 rable individuals to which collectively the title of " com- 

 pound ciliated gemmules " or " swarm -gemmules " may be 

 most appropriately applied. The chain of evidence support- 

 ing this decision, constructed out of the ample data yielded by 

 the investigations of the several specialists mentioned, col- 

 lated with my own in the same direction, may now be sub- 

 mitted. 



The initial term or starting-point of the so-called 

 ciliated sponge-embyro is generally recognized as con- 

 sisting of a small unicellular Avioeba-like unit possessing 

 the faculty of locomotion from place to place by the protru- 

 sion of lobate pseudopodia after the manner of a typical 

 Amoeba. The diameter of the smallest of these initial units 

 averages the 3000th part of an English inch, its appearance 

 corresponding with that given at PI. VI. fig. 2. From 

 this most minute size these initial factors occur in every gra- 

 dation to the dimensions of about the 200th part of an inch, 

 under which larger proportions a spheroidal quiescent state 

 is assumed and the first metamoi'phosis commences. This is 

 eiFected by the symmetrical cleavage or duplicative division 

 transversely and longwise, first into two, then successively into 

 four, eight, sixteen, thirty-two segment-masses, and so on, of 

 the entire spheroidal protoplasmic mass. The final result of this 

 continued process is the production of a spherical aggregation 

 of minute rounded units or segment-spheres, agreeing, to all 

 appearance, with the morula derived from the segmentation or 

 cleavage of the ovum of all ordinary higher animals or Meta- 

 zoa. Figs. 3 to 8 of PL VI. serve to illustrate the leading 

 phases of this transformation. According to Mr. Carter the 

 foregoing process of cleavage takes place within a hyaline 

 investing envelope ; but the existence of such a structure is 

 not confirmed by the investigations of Haeckel, Barrois, or 

 myself. The next characteristic phase, universally conceded, 

 is the assumption by the morula-like body of a more or less 

 ovate outline, accompanied by the clothing of the entire peri- 

 pheral surface with long vibratile cilia or flagella. This peri- 

 pheral surface viewed superficially presents under high mag- 

 nification a tessellated aspect, each minute polygonal area of 

 this tessellation representing the external or exposed surface of 

 one only of the innumerable segments into which the primary 

 unicellular body has been divided. PL VI. fig. 9 represents 

 such a superficial view, the cilia round the margin of the 

 organism, for the sake of perspicuity, being alone introduced. 

 Focusing a little deeper, so as to bring into clear view the 



