EAEE FOEM OF TFIE BLASTODERM OF THE CHIOK. 879 



the yolk, and the same is still true in the mounted prepara- 

 tion. By transmitted light I can see with the unaided eye the 

 primitive streak and the external arm of the primitive groove 

 as a clear line leading from the notch to the base of the streak. 

 The inner arm of the groove is just barely recognisable under 

 the same conditions. The external arm is a little more than 

 twice the length of the inner arm, and forms with the latter 

 an angle of about 110°. In the blastoderm found by Dr. 

 Bohm, the posterior portion of the groove formed a straight 

 line with the anterior, and thus the marginal notch fell 

 directly behind the primitive streak. 



What now is the meaning of this continuity between the 

 primitive groove and the marginal notch ? It is certainly a 

 deviation from the avian type of development, of such rare 

 occurrence that it must be admitted to be an anomaly ; still it 

 is, if I am not mistaken, an anomaly for which comparative 

 embryology furnishes a very satisfactory explanation. It is a 

 well-known fact that the embryos of all Amniota are formed 

 near the centre of the blastoderm, while those of all anamni- 

 otic vertebrates have their origin at the margin. It is now 

 generally believed that the central position has been derived 

 secondarily from the marginal, and this belief has given rise to 

 various speculations in regard to the meaning of the primitive 

 streak and its relation to the embryo. Intimately connected 

 with these questions is another relating to the mode of forma- 

 tion and the growth in length of the embryo. 



That these questions are not very easy of solution is evident 

 from the fact that, after half a century of patient research, a 

 decade or more of plodding " microtomization," and the indus- 

 trious accumulation of embryometrical tables as the fruit of 

 sections made in every nameable plane, the most eminent 

 authorities in embryology cannot agree in their interpretation 

 of the primitive streak, nor even in an opinion as to the part it 

 plays in the formation of the embryo. When Kupffer and 

 E-auber differ as to what constitutes the blastopore of the 

 chick ; when Goette, Kolliker, His, and Waldeyer affirm that 

 the primitive streak is directly concerned in the formation of 



