452 THE INVERTEBRATA 



In the numerous cases in which no tympanum capable of respond- 

 ing to sound waves exists, a precise function is not clearly indicated. 

 According to some, they may act as rhythmometers, i.e. co-ordinators 

 of the rhythmical movements of the insect's body. A more probable 

 function is that of perceiving vibratory stimuli from without. 



The organs of vision have been dealt with in Chapter x, and it is 

 perhaps enough to mention that the ommatidia of which the com- 

 pound eye is built up, are specialized sensillae of hypodermal origin, 

 essentially similar to those already mentioned. 



Embryology . Though Arthropod eggs vary in the amount of yolk 

 contained within them they are for the most part yolky and are 

 ceiitrolecithal in type (p. 316). To this feature must be ascribed those 

 distorting influences which make Arthropod development so different 

 from that of other invertebrates. 



Among insects it is only in the primitive Apterygota and in many 

 parasitic Hymenoptera that are found small, comparatively yolkless 

 eggs which undergo total cleavage. But though these may represent 

 the primitive condition, they cannot be taken as typical of modern 

 insects. 



The typical 3^olky egg is provided with a vitelline membrane and 

 a stout chorionic shell which is commonly sculptured. After fertiliza- 

 tion, incomplete cleavage sets in, a process involving only the suc- 

 cessive mitoses of nuclei. In this early stage, therefore, the egg is a 

 syncytium of very yolky cytoplasm in which lie the cleavage nuclei. 

 These wander to the peripheral cytoplasm, there to form an outer 

 cellular layer or blastoderm (Fig. 316 A and B). In this latter, occurs 

 a thickening, thus separating embryonic from extra-embryonic 

 blastoderm and in its relation to the yolk the embryo now resembles 

 an inverted chick embryo, but, as might be expected, its method of 

 differentiation is highly different. 



Gastrulation proceeds as follows. From the middle line of this 

 embryo certain cells pass inwards towards the yolk by invagination, 

 by proliferation or by their overgrowth by cells of the germ band 

 lateral to them. This enclosed cell mass is mesoderm (together with 

 endoderm in certain cases). The plate left outside constitutes the 

 ectoderm (Fig. 316 C and D). In such cases where endoderm is not 

 included in the enclosed mass as above, this layer rises from growth 

 centres, anterior and posterior, at the places where the stomodaeum 

 and proctodaeum will appear or already have differentiated. The result 

 in any of these cases is a three-layered embryo relegated to the ventral 

 side of the egg, i.e. beneath the yolk. It consists of a layer of outer 

 ectoderm, within which is the mesoderm from which segmental 

 somites develop. Against the yolk lies the endoderm destined to form 

 the mid gut. The mesoblastic somites give rise on their upper borders 



