12 Growth in length of the Vertebrate Embryo 



which at first appeared almost straight and transverse, soon 

 lengthens and becomes crescentic, and finally its horns meet and 

 we have a white patch of larger segments surrounded by a dark 

 sharply defined rim. The white patch is formed by a plug of 

 yolk cells which almost completely blocks the blastopore, but all 

 round the plug the archenteric cavity is visible as a chink between 

 the plug and the lip rim. This rim is the blastopore lip. 



Examination of vertical sections, Fig. 7 E H, shows that a 

 narrow slit, continues inward from the groove parallel with the 

 surface and some half-a-dozen cells below it. The slit gradually 

 lengthens inwards from the groove and eventually expands into 

 a cavity as the blastopore forms and closes. This cavity is the 

 future gut cavity, and it may for the moment be called archen- 

 teron, though as we shall see it corresponds to rather more than 

 what we shall eventually term archenteron. 



By this act of splitting the gut cavity is initiated, and the 

 formation of the two primary layers is established, firstly in the 

 region of what will be the future dorsal surface, then in the lateral 

 and lastly in the ventral region. 



The process of gastrulation is complicated in the Amphibia 

 by the fact that the gut cavity is formed dorsally a considerable 

 time before it is formed ventrally or perhaps it would be more 

 correct to say that it is formed anteriorly sooner than posteriorly. 

 It is still further complicated by the origin of the process of growth 

 in length which also begins first on the dorsal side. 



Experimental observations show quite unmistakeably that as 

 soon as the blastopore lip is formed it begins to grow over the 

 yolk, and does not cease to grow in the dorsal region until the full 

 complement of segments have been formed. The blastopore lip con- 

 stitutes a ring-like area of proliferating tissue which is continuously 

 adding on new material to all previously existing material with 

 which it is in contact. It forms the deuterogenetic centre. 



The series of sagittal sections, shown in Fig. 7, indicate the 

 character of the changes which take place internally and are 

 accompanied by surface views. 



The slit seen in F running in from the groove shown in surface 

 view in B widens out in G and H to form a capacious chamber 

 which is the archenteron the first formed part of the gut cavity. 



