XIII 



PHYLUM CHORDATA 239 



on the other to the next class to be studied the Amphibia. The 

 ova become enclosed, while passing down the oviduct, in a gela- 

 tinous envelope which swells up considerably when it comes in 

 contact with the water. At what stage fertilisation takes place 

 is not exactly known. Segmentation is complete and unequal, 

 and results in the formation of a lens-shaped blastula (A) with 

 smaller cells on one of the convex surfaces (the future dorsal) and 

 larger on the other (the future ventral). A blastopore (U. p.) 

 first appears on the ventral surface as a short transverse slit, 

 which grows into a semicircle (S) or a horse-shoe. The free ends 

 of this grow in towards one another and unite to enclose an 

 irregularly circular or elliptical space filled in by a mass of large 

 cells the yolk-plug (G yk. pl.\ Soon, however, this wide aperture 

 becomes narrowed to a small longitudinal slit, the lips of the 

 anterior part of which soon unite to form a longitudinal seam or 

 suture only the most posterior part remaining open (D). During 

 its increase in size the blastopore has been growing over toward, 

 the dorsal side, and when its lips become united to form a suture it 

 extends along the greater part of the dorsal surface. A pair of 

 medullary folds appear at the sides of this (E) and are coalescent 

 in front of it. From the medullary folds and the groove between 

 them the neurocoele, and subsequently the entire nervous system, 

 are developed as in Craniata in general (see p. 92). The portion 

 .of the blastoderm destined to give rise to the embryo becomes to 

 a slight extent folded off from the rest, which forms an ill-defined 

 rounded mass or yolk-sac to be subsequently absorbed as develop- 

 ment proceeds. The most important features in the later stages 

 (F) are the negative ones of the absence of the external gills (to 

 be referred to subsequently in the account of the Amphibia) and 

 the absence of horny jaws. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Dipnoi are Pisces in which the notochord is persistent, and 

 the primary cranium persists with little ossification, but has added 

 to it a number of membrane-bones. The skull is autostylic, the 

 lower law articulating with a palato-quadrate process which is 

 immovably fixed to the side of the skull. There are four or five 

 cartilaginous branchial arches. The dermal fin-rays are horny in 

 character and are supported by numerous cartilaginous pterygio- 

 phores The caudal fin is diphycercal. The paired fins are ot the 

 character of archipterygia. The pectoral arch is a single cartilage 

 with a pair of superficial membrane-bones. The pelvic arch is 

 well-developed and cartilaginous. There are gills attached to the 

 branchial arches, and in addition a single or double lung opening 

 into the pharynx. The gills are covered over by an operculum 

 'There is a dermal skeleton in the form of overlapping cycloid 



