86 MR. NOEL TAYLER ON A UNIQUE CASE OF 



embryonal rudiments were represented by two primitive streaks 

 about equally developed ; while other and older stages have been 

 recorded by Kaestner and others (6, 7). 



Under the circumstances however, the question does not seem 

 one of great importance, and it seems better to refrain from any 

 definite decision as to the orientation of Rudiment /3. 



No structure seems to be present which can undoubtedly be 

 regarded as a " primitive knot." 



We may now turn to the morphological relationships of the 

 two embryonal formations. These two structures occurring in a 

 single blastoderm, the three primary germ-layers of each and 

 those of the extra embryonal portion of the blastoderm, are of 

 course continuous each with each. It may also be noticed that, 

 whilst the sinus terminalis is perfectly regular in outline, the area 

 pellucida shows signs of division into two regions of greater trans- 

 parency (see PI. I. fig. 1) in relation to the two embryonal forma- 

 tions, since the blood islands of the vascidar area are concenti-ated 

 somewhat in the regions posteriorly and postero-laterally to 

 Rudiment /3. It is of more importance, however, to find that 

 there is what may be described as a definitely differentiated 

 morphological connection between the two embryonal formations, 

 viz., the rather darkly staining and irregular strand which in the 

 photograpli of the whole mount (PI. I. fig. 1) appears to run in the 

 long axis of ♦ he blastoderm between the two embryonal rudiments. 

 In tlie sections this is found to consist of a narrow and somewhat 

 irregular band of unsplit mesoderm, thicker and broader in the 

 legion of the Rudiment ft and becoming more tenuo\is and 

 iiiegular as it comes into proximity to Rudiment a. 



Its somewliat inegvilar and broken appearance in the whole 

 mount is seen from the sections to result from the presence of 

 vessel -like Incunfe in its svibstance, spaces, however, which are 

 devoid of blood corpuscles. 



On either side of this median strand the mesoderm splits into 

 somatic and splanchnic layers, and it would seem that the extra- 

 embryonal ccelom situated laterally to Rudiment ft and to the 

 mesodei-mal strand is continuous with the parietal coelom of 

 Rudiment o. 



C. 7Vie Morjyhology of the greater Emhryonal Formation 

 (Rudiment a). 



Proceeding to a consideration of the structure of Rudiment o, 

 the moi-phology of the posterior region may be briefly dismissed 

 as it is that of a normal embryo of 17 somites. PI. I. fig. 3 and 

 text-fig. 1 represent a semi- diagrammatic figure of the morphology 

 of the anterior region, while transverse sections through the more 

 important regions ai*e shown in Plates II. and III. figs. 4 & 5. 

 The positions of these transverse sections are indicated in text- 

 fig. 1. 



In section D, p. 91, is given a concise summary of the more im- 

 portant morphological abnormalities which occur in Rudiment a, 

 and this is subdivided into four categories, viz., those afl^'ecting 



