AVES. 137 



through the whole period of the early development. The third passage is 

 formed in tlie chick (Ku])ffer) during the third day of incubation. In 

 Melopsittacus undulatus the two first communications are stated by Braun 

 (No. 120) to be present at the same time, the one iu 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 1 

 blastopore in Eiasmobranchii which becomes converted into the tail-swelling 1 

 and the lining of the neurenteric 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 Eiasmobranchii, which shuts ofl' the embryo from the edge of the 

 blastoderm {vide p. 52), though there is of course no genetic relation between 

 the two structures. When the anterior part of the sti'eak is becoming 

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

 shallows and finally disappeai"S. The hinder part itself atrophies from 

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

 the yolk the j^art 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'. 



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- 

 |)robable 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 

 through the primitive streak. Whei-e the primitive streak becomes divided 

 into two parts the sections through the two parts are given separately : the 

 left column (A) referring to the anterior part of the streak ; the right 

 column (P) to the posterior jiart. The third column gives the number of 

 sections through the embryo. The first table is for fowl embryos, the 

 second for goose embryos. 



^ This nomenclature may seem a little paradoxical. But 011 reflection it will appear 

 that so long as the embiyo 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 fairly 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 ouglit in con- 

 tinuing 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 rice (vcsa. T 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 eveiything quite clear. The heart of 

 Acipenser (Itt) 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 hir.d end, and vice versa. 



