No. 3-] THE EMBRYOLOGY OF A TERMITE. 535 



meiit of the ancestral insects. The facts of the development 

 of the Crustacea, Palaeostraca, the Arachnids, and the Myrio- 

 pods (see Korschelt and Heider (17), p. 741) show a similar 

 disproportion in size between the primary rudiment and the 

 definitive segmented adult. This may be illustrated from the 

 Arthropods by referring to the growth of a Nauplius into its 

 adult form. A similar method of growth is found in the develop- 

 ment of the Annelid from the Trochophore, where also growth 

 is uncomplicated by the presence of yolk. I do not mean to 

 raise any question of homology between the primary disc-shaped 

 rudiment of the insect embryo, and either the Nauplius or the 

 Trochophore, but to point out that a certain few insects (Ter- 

 mites, etc.), otherwise primitive, have retained a method of growth 

 (see closing paragraphs of this paper) fundamentally similar to 

 that followed by other segmented forms. In most insects, and 

 particularly in the more specialized forms, the formation of a 

 segmented embryo is more direct, a rather long germ-band being 

 established from the first (and, as I take it, precociously), of 

 more nearly the definitive length of the embryo. {Note that 

 Graber s (g) classification of germ-bands is not here accepted.) 



C. These primitive forms (Orthoptera and Termite) are also 

 characterized by another peculiarity of interest in the present 

 discussion. The amnion arises very early and completely covers 

 the embryo soon after its appearance as a synall disc. We do 

 not know with certainty to what need of the embryo the amnion 

 responds, but we are not surprised to find it in its most primi- 

 tive condition in the very forms under consideration, which are 

 primitive in so many other morphological characters. I believe 

 this is the case, and that insects, in which the membranes becom,e 

 prominent and cover over the embryo comparatively late in its 

 growth, represent a secondary condition. If, as is generally 

 supposed, the amnion arose as a protection for the germ-band 

 against mechanical injury or too rapid evaporation, or as a sac, 

 to receive accumulated waste products,as Wheeler (25) suggested, 

 it would have been a great advantage for it to appear in the 

 ancestral Pterygota at the earliest possible moment in the 

 growth of the embryo. This moment occurs when the first 

 rudiment of the embryo, the germ-disc, is established and about 



