210 POL YPLA COPHORA . 



man tie- fold in which the shell of the adult Aplysia and its congeners is 

 concealed. The pen, then, of Cephalopoda would not represent the plug of 

 the shell-gland. In regard to this view of the case, it may be remarked 

 that I have found no trace in the embryonic history of the living 

 Dibranchiata of a structure representing the phragmacone ; and further, it 

 is possible, though little importance can be attached to this suggestion, that 

 the Dibranchiate pen-sack, as seen in its earliest stage in the embryo Loligo, 

 etc., is fused with the surviving remnants of an embryonic shell-gland. 

 When the embryology of Nautilus pompilius is worked out, we shall pro- 

 bably know with some certainty the fate of the Molluscan shell-gland in the 

 group of the Cephalopoda." 



The funnel. The general development of the funnel has already 

 been sufficiently indicated. The folds of which it is formed are 

 composed both of epiblast and mesoblast. The mesoblast of the 

 anterior part of each half of the funnel would appear to give rise to 

 a muscle passing from the cartilage of the neck to the funnel proper. 

 The posterior parts gradually approximate, but meet in the first 

 instance ventrally. The two folds at first merely form the side of a 

 groove or imperfect tube (fig. 113 C and 124 ff.), but soon the free 

 edges unite and so give rise to a perfect tube, the primitive origin 

 of which by the coalescence of two halves would not be suspected. 

 In Nautilus the two halves remain permanently separate but over- 

 lap each other, so as to form a functional tube. 



Polyplacophora. The external characters of the embryo of Chiton 

 have long been known through the classical observations of Loven 

 (No. 285), while the formation of the layers and the internal pheno- 

 mena of development have recently been elucidated by Kowalevsky 

 (No. 284). The eggs are laid in April, May, and June, and are en- 

 closed in a kind of chorion with calcareous protuberances. The seg- 

 mentation remains regular till sixty-four segments are formed. The 

 cells composing the formative half of the ovum then divide more 

 rapidly than the remainder ; there is in this way formed an elongated 

 sphere, half of which is composed of small cells and half of larger 

 cells. In the interior is a small segmentation cavity. From its 

 eventual fate the hemisphere of the smaller cells may be called the 

 anterior pole, and that of the larger cells the posterior. An involu- 

 tion of the cells at the apex of the posterior pole (though not of the 

 whole hemisphere of larger cells) now takes place, and gives rise 

 to the archeiiteron. At the same time an equatorial double ring of 

 large cells appears on the surface between the two poles, which be- 

 comes ciliated and forms the velum. At the apex of the anterior 

 pole a tuft of cilia, or at first a single flagellum, is established (tig. 

 116 in. and IV.). 



In the succeeding developmental period the blastopore, which 

 has so far had the form of a circular pore at the posterior extremity 

 of the body, undergoes a series of very remarkable changes. In con- 

 junction with a gradual elongation of the larva it travels to the ventral 

 side, and is prolonged forwards to the velum as a groove. The 



