AVES. 165 



Melopsittacus undulatus the two first communications are stated by Braun 

 (No. 120) to be present at the same time, the one in front of the other. 



It is probable, from the above description, that the front portion of the 

 primitive streak in the bird corresponds with that part of the lips of the 

 blastopore in Elasmobranchii which becomes converted into the tail-swelling 

 and the lining of the neurentic canal ; while the original groove of the 

 front part of the primitive streak appears to be converted into the posterior 

 diverticulum of the neural canal. The hinder part of the primitive streak 

 of the bird corresponds, in a very general way, with the part of the blasto- 

 pore in Elasmobranchii, which shuts off the embryo from the edge of the 

 blastoderm (vide p. 64), though there is of course no genetic relation between 

 the two structures. When the anterior part of the streak is becoming 

 converted into the tail-swelling, the groove of the posterior part gradually 

 shallows and finally disappears. The hinder part itself atrophies from 

 behind forwards, and in the course of the folding off of the embryo from 

 the yolk the part of the blastoderm where it was placed becomes folded in, 

 so as to form part of the ventral wall of the embryo. The apparent hinder 

 part of the primitive streak is therefore in reality the ventral and anterior 

 part 1 . 



It has generally been maintained that the primitive streak and groove 

 become wholly converted into the dorsal portion of the trunk of the embryo, 

 i.e. into the posterior part of the medullary plate and subjacent structures. 

 This view appears to me untenable in itself, and quite incompatible with 

 the interpretation of the primitive streak given above. To shew how im- 

 probable it is, apart from any theoretical considerations, I have compiled 

 two tables of the relative lengths of the primitive streak and the body of 

 the embryo, measured by the number of sections made through them, in a 

 series of examples from the data in Gasser's important memoir (No. 127). 

 In these tables each horizontal line relates to a single embryo. The first 

 column shews the number of somites, and the second the number of sections 



1 This nomenclature may seem a little paradoxical. But on reflection it will 

 appear that so long as the embryo is simply extended on the yolk-sphere, the point 

 where the ventral surface begins has to be decided on purely morphological grounds. 

 That point may faidy be considered to be close to the junction of the medullary plate 

 and primitive streak. To use a mathematical expression the sign will change when 

 we pass from the dorsal to the ventral surface, so that in strict nomenclature we 

 ought in continuing round the egg in the same direction to speak of passing backwards 

 along the medullary, but forwards along the primitive streak. Thus the apparent 

 hind end of the primitive streak is really the front end, and vice versA. I have 

 avoided using this nomenclature to simplify my description, but it is of the utmost 

 importance that the morphological fact should be grasped. If any reader fails to 

 understand my point, a reference to fig. 52 B will, I trust, make everything quite 

 clear. The heart of Acipenser (ht) is there seen apparently in front of the head. It 

 is of course really ventral, and its apparent position is due to the extension of the 

 embryo on a sphere. The apparent front end of the heart is really the hind end, and 

 vice versa. 



