2 OUTLINES OF CHORDATE DEVELOPMENT 



"As an introduction to the study of embryology, and as an 

 indispensable aid to a reasonable appreciation of the value 

 of embryological facts, the life-history of Amphioxus 

 provides an object which ... is perhaps unrivalled. 

 It is alike useful in a text-book of human embryology, and 

 in one of invertebrate zoology." (Willey, "Amphioxus," 

 etc., p. 104.) 



MOKPHOLOGICALLY Amphioxus (Branchiostoma lanceolatum) 

 is recognized as one of the important types of Chordata, for it 

 illustrates in simple form the essential characteristics of this 

 large group, most of the members of which are extremely com- 

 plex. And in its mode of development, no less than in its struc- 

 ture, Amphioxus serves as a key to the more complicated con- 

 ditions of the Craniate groups. The anatomical and embryolog- 

 ical simplicity of this creature is commonly regarded as an 

 indication of true primitiveness, although the morphologist 

 recognizes, as the embryologist must also, that the simplicity 

 of primitiveness is here obscured, in many respects, by conditions 

 which are obviously special adaptations or secondary alterations 

 of primary arrangements. 



Many of the simple embryological characteristics of this form 

 are correlated with the freedom of the egg from a large yolk- 

 mass. This is equivalent to saying that the accumulation of 

 yolk in the eggs of most Chordates is a secondary character, and 

 is, to a considerable extent, the cause of many of those modifica- 

 tions of the course of development which lead to unusual con- 

 ditions, proving difficult of comparison with other develop- 

 mental types. 



The egg of Amphioxus is small, and the deutoplasm, small in 

 amount, is scattered through its substance. In correlation 

 with this, cleavage is total and quite regular in its course, leading 

 to the formation of fairly typical blastula and gastrula. The 

 organ-rudiments are all formed first as simple epithelial struc- 

 tures, whose origins and fates are easily followed, for each part 

 is sharply outlined and remains clearly demarcated through 

 the course of its development. The embryo is free swimming 

 and externally ciliated. In all these respects Amphioxus illus- 

 trates primitive Chordate characteristics. Most of the second- 



