OUTLINES OF THE DEVELOPMENT OF 'HBK TUATARA. 17 



ing granular mass, in which it is impossible to make out the 

 outlines of the cells at all. Between this and the primitive 

 streak its cells show a prismatic arrangementj and may be 

 regarded as forming a medullary plate (fig. 4, M. P.). Just 

 in front of the blastopore, especially a little to each side of the 

 middle line, it shows a strong tendency to separate from the 

 layer beneath it. 



The inner layer (P. S. M.) of cells (mesoblast), continued 

 forwards from the primitive streak beneath the epiblast, gra- 

 dually thins out till it reaches a point a little in front of the 

 middle of the embryo, where it is actually thinner than the 

 overlying epiblast; it then thickens again considerably in the 

 neighbourhood of the head-fold, and finally contracts oiice 

 more to become continuous with the lower layer [L. L.) of the 

 area pellucida. 



Behind the blastopore the primitive streak passes gradually 

 into a thick mass of precisely similar cells, which can be traced 

 backwards for some distance beneath the epiblast of the area 

 pellucida. This mass of mesoblastic cells (fig. 4, Si.) forms an 

 ill-defined transverse thickening behind the blastopore, and 

 evidently represents the so-called " sickle '^ of other reptilian 

 embryos. 



It will be observed that no amnion is yet present. It is 

 true the section represented in fig. 4 shows an uprising of the 

 blastoderm (pro-amnion) in front of the head-fold, but nothing 

 was visible in the living embryo, and I have no doubt the 

 appearance shown is simply due to contraction. 



Stage D (figs. 6—14). 



I have obtained only a single example of this stage — viz. 

 embryo 58, removed on December 9th, from an egg found by 

 Mr. Henaghan about the end of November. 



The external appearance of this embryo, when viewed from 

 above as an opaque object without staining, is shown in fig. 6. 

 No trace of the amnion is yet visible. The head-fold still 

 projects conspicuously above the blastoderm, and has grown 

 forwards, while its anterior end has become considerably nar- 



VOL. 42j PART 1. — NEW SEE. B 



