A AMPHINEUKA. 



I. The Development of Chiton. 



The external characters of the larva of Chiton and the develop- 

 ment of the same were described many years ago by LOVEN 

 (No. 10), while the formation of the germ-layers and the various 

 internal changes have since been investigated with the assistance 

 of modern methods by KOWALEVSKY (Nos. 6-8).* 



Oviposition and Nature of the Eggs. The eggs pass from the genital 

 duct into the mantle-groove of the female in an immature condition, 

 and are there fertilised by the sperm which the male has discharged 

 into the water ; after this they are deposited either singly or in groups. 

 They are found floating freely in the water; but, in some cases, as 

 in C. Polii, the eggs are retained, until the embryo is mature, 

 within the mantle-cavity of the female. Each egg is enclosed 

 within a spiny shell, the surface of which is marked out into poly- 

 gonal areas. The form of these spines varies in the different species 

 and genera. The egg itself does not appear to be very rich in yolk. 



1. The Cleavage and the Formation of the Germ-Layers. 



The cleavage is total and practically equal, the egg being divided 

 lay two meridional grooves first into two and then into four blastomeres 

 of almost equal size. At the eight- blastomere stage, however, a 

 slight differentiation of the cleavage-spheres lying at the animal pole 

 takes place, four larger (vegetative) and four smaller (animal) blasto- 

 meres being distinguishable. The animal pole is marked by the 

 polar bodies which lie almost exactly over the point of intersection of 

 the cleavage-planes. During the further course of cleavage, the 

 blastomeres of the vegetative half are at first to be distinguished by 

 their larger size, the smaller size of those of the animal pole being due 

 to their more rapid division. In the later stages, as in the earliest, 

 there is also a certain resemblance to the conditions found in the 

 Gastropoda which are characterised by a stage consisting of four 

 macromeres and four micromeres (Fig. 40 C, p. 109). We find here the 



gastres are Mollusca at all. According to SEDGWICK, the most important 

 difference between the Solenogastres and all other Mollusca is that in the 

 former the gonad opens directly into the pericardium. This distinction does 

 not appear to us to be so vital when we remember that in some Molluscs the 

 gonad communicates directly with the kidney, the latter in, turn opening into 

 the pericardium, and further that the cavities of all three organs are parts of 

 the primitive coelom. ED.] 



* [METCALF (No. I.) has since reinvestigated the embryology of Chiton, pay- 

 ing special attention to the cleavage and cell-lineage, his observations entirely 

 confirming those made by KOWALEVSKY. ED.] 



