274 



LECTURE XX. 



Larva of Ama- 



roiiciuni proli- 



fei'ura. 



wards has observed the contact of the s^iermatic animalcule with the 

 ova in the cloaca. 



The next stage which he records, viz. the granular or 

 mulberry structure of the vitellus, is the result of the 

 spontaneous divisions and assimilative powers of the 

 yoke-cells. The subdivided mass through which the 

 properties of the hyaline and fertilising principle have 

 been diffused, is next covered by the expansion of the 

 germ-spot, forming the basis of the integument. A 

 process of this integument then begins to extend from 

 a particular point, and, rapidly elongating, wraps itself 

 like a cord about the vitellus. This body with its in- 

 tegument then becomes condensed, and separates from 

 the chord, which, retaining only its basal attachment to 

 the pellucid integument, forms the caudal appendage. 

 The integument (Jtff. 117' a) increases in thickness. 

 The extremity of the yolk opposite the caudal attachment 

 developes a series of cylindrical productions, which re- 

 minds one of the arms of a polype, but they are few in 

 number. Three of them have expanded extremities 

 (b^^, b'^), which increase in length ; whilst the other processes di- 

 minish, and finally disappear. A spiral filament is continued from 

 the membrane of the vitellus down the centre of the tail (b'). 



In this state the embryo escapes from the ovum, generally while in 

 the cloaca of the parent, but sometimes after the egg has been ex- 

 pelled from the common central outlet. The young animal immedi- 

 ately unfolds its tail, and begins to swim like the tadpole of the frog, 

 which it so much resembles in form. The three clavate cephalic 

 processes are the organs by which it effects its final adhesion and 

 settlement. When this has taken place, the tail shrinks, and is 

 usually detached by progressively increasing contraction at its base ; 

 — a kind of spontaneous fission. 



The sessile and adherent trunk now becomes the seat of an active 

 development: the integument is thickened; the yolk becomes elon- 

 gated and divided by a circular constriction into two unequal parts, 

 in each of which a clear spot can be recognised. One of these spots, 

 by subsequent development, becomes the heart ; the other the res- 

 piratory sac. The subdivided vitelline mass, which now begins to be 

 rapidly metamorphosed into the special tissues, also acquires a distinct 

 tunic, which soon separates itself from the thick and gelatinous ex- 

 ternal integument. The quadrifid orifice of the branchial sac is first 

 formed upon the internal tunic. The contour of the great resj^iratory 

 pharynx can next be discerned, and the constriction of the sac opposite 



