THE HATCHING SPINES 



585 



The meat-fly, as we have observed, hatches in the following 

 manner. The embryo moves to and fro, the body twisting until 

 the exochorion is ruptured; the egg-shell splits longitudinally, and 

 in one or two seconds the larva pushes its way out through the 

 anterior end, and in a second or two more extricates itself from the 

 shell. The latter scarcely changes its form, and the larva slips out, 

 leaving the amnion within. 



In the case of a fossorial wasp, Specius speciosus, which carries 

 Cicadae into its burrow, laying an elongated egg on the body under 

 the median thigh of its victim, the larva on hatching, Riley states, 

 " does not emerge from the skin of the egg, but 

 merely protrudes its head and begins at once to 

 draw nourishment from between the sternal 

 sutures of the Cicada." 



The hatching spines. Animals belonging to 

 quite distinct classes are provided late in em- 

 bryonic life with hard knobs or spines, which 

 are temporary structures for the purpose of 

 breaking or cutting open the egg-shell, when it 

 is too thick and solid to be ruptured by the 

 movements of the embryo. The embryos of cer- 

 tain lizards, turtles, the blind worm and some 

 snakes, of the crocodile, and even birds, as well 

 as the duckbill and Echidna, are provided with 

 them, always occurring, so far as we are aware, 

 on the end of the upper jaw. In the Arthropoda 

 similar structures have thus far only been met 

 with in myriopods and insects, though an anal- 

 ogous structure on the cephalothorax of the 

 embryo of phalangids has been observed by 

 Balbiani. Metschnikoff describes and figures 

 a low conical spine serving this purpose situated on the embryonal 

 cuticle over the head of the advanced embryo of Strongylosoma, 

 and one on the 3d pair of mouth-parts of Geophilus. 



In the winged insects, the embryo of Forficula is said by Heymons 

 to bear a single spine between the eyes, which serves as an egg- 

 tooth. The embryo of the Hemerobidee, according to Hagen, " opens 

 the egg with an egg-burster like a saw." (Proc. Bost. Soc. Nat. 

 Hist., xv, p. 247.) Riley states that the egg-burster, or ruptor 

 ovi, as he calls it, of Corydalus cornutus, has "the form of the com- 

 mon immature mushroom," and he adds that it is a part of the 

 amnion, being "easily perceived on the end of the vacated shell." 



B 



FIG. 554. Egg-case of 

 Mantis with young escap- 

 ing : A, the case with 

 young in their position of 

 suspension. ,cerci mag- 

 nified, showing the sus- 

 pensory threads. After 

 Brongniart, from Sharp. 



