46 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



cavity, the archenteron, which runs forwards from the mid-posterior 

 margin. In the roofing in of this archenteron it is apparently a 

 process of overgrowth which plays the main part — but along the rest 

 of the blastoderm margin the process of overgrowth appears to die 

 away and its place is taken by delamination very much as was the 

 case in Lepidosiren. This is shown by the fact that the invagination- 

 groove, which, as already remarked, extends outwards on each side 

 for some distance, never deepens to any considerable extent except 

 in its middle part. 



In the region in front of the archenteron the deeper or lower 

 layer cells of the blastoderm increase greatly in number and spread 

 forwards so as gradually to fill up the segmentation cavity. The 

 remains of the latter persist longest near the anterior margin and 

 the ectoderm covering the last remnant of the segmentation cavity 

 commonly projects as a small but conspicuous elevation above the 

 general surface of the blastoderm. 



These lower cells eventually take on a mesenchymatous character 

 for the most part. Those lying next the yolk-syncytiuni however 

 give rise to a definite epithelium, known as the yolk epithelium. 

 Some of them are said to penetrate actually into the yolk where 

 their nuclei assume the characters of the nuclei of the yolk-syncytium. 

 The floor of the archenteron is formed by the yolk epithelium which 

 is continubus round the inner, or anterior, end of the archenteric 

 cavity with the endoderm of its roof. 



Actinopterygii. — It is unfortunate that in the more familiar 

 Actinopterygians belonging to the group Teleostei — of which it is 

 so easy to obtain developmental material — the phenomena of gastru- 

 lation are obscure and their investigation is impeded by technical 

 difficulties in the way of making satisfactory sections. "We shall 

 therefore confine ourselves to indicating in a few words the more 

 conspicuous features of the process. 



On the whole the features of gastrulation closely resemble those 

 met with in Elasmobranchs — a resemblance which however we are 

 not justified in regarding otherwise than as a phenomenon of con- 

 vergence, seeing that the general evidence of morphology points 

 to the ancestors of the Teleosts being much more closely related to 

 the holoblastic Ganoids than to the existing Elasmobranchs. A 

 characteristic feature to be noted is that here, as will be found to be 

 the case in various mammals, the superficial cells of the blastoderm 

 become much flattened and form a thin protective covering layer 

 which takes no part in the development of the embryo. 



When gastrulation is commencing the posterior margin of the 

 blastoderm presents in longitudinal vertical sections the appearance 

 of being turned inwards to form the two primary layers. There is 

 no actual patent archenteric cavity though the inflected portion 

 clearly represents the archenteric roof, the floor being apparently 

 represented by the underlying syncytial layer. 



The growth in length of the archenteric roof seems to be brought 



