186 



INVEETEBRATA 



CHAP. 



Pig. 134. — Two views of developing eggs ot Astacus 

 Jluviatilis seen from the ventral surface. (After 

 Relchenbach.) 



A, stage in which the rudiments of maxillae have appeared, 

 and in which the caudal fork is visible. B, stage in which the 

 rudiments of thoracic appendages are appearing, and in which 

 theabdomeu is segmented, ah, abdomen ; atX, first antenna ; at^.g, 

 antennulary ganglion (deuterocerebrum) ; at^, second antenna ; 

 car, fold which becomes edge of the carapace ; caud.f, caudal 

 fork ; eg, cerebral groove wliich gives rise to the optic ganglion ; 

 hib, labrum ; mTi, rudiment of mandible ; wu:l, first maxilla ; 

 mx^, second maxilla ; mxp^, imx}^, vixp^, first, second, and third 

 maxillipedes ; oc, eye-stalk ; op,g, optic-ganglion ; pr.c, proto- 

 cerebrum ; ret, retinulae of the compound eye ; tli, rudiments 

 of thoracic appendages. 



original three have be- 

 come longer ; and that 

 the second, which is the 

 rudiment of the antenna, 

 has become bifurcated at 

 the end, which is an in- 

 dication of the branching 

 of the limb into exopo- 

 dite and endopodite. 

 Behind the mandible is 

 the region of the ventral 

 shield, which owes its 

 origin to this budding 

 zone ; on it are to be 

 found the rudiments of 

 five new pairs of appen- 

 dages, viz. those corre- 

 spending to the first and 

 second maxilla and to 

 the three pairs of maxil- 

 lipedes. All except the 

 first of these are very 

 faintly marked indeed. 



The thoracico- 

 abdominal rudiment has 

 grown in length and has 

 become marked out into 

 segments by grooves. On 

 the dorsal side, just at 

 its point of origin, there 

 is seen an ectodermic 

 thickening. Below this 

 there is a plate-like mass 

 of mesoderm, the ends of 

 which in the next stage 

 become bent upwards 

 and attached to the ecto- 

 derm, so as to enclose a 

 space which is the cavity 

 of the heart (H, Fig. 

 135). In the sides of 

 this mesodermal mass, 

 just as in the case of 

 Peripatu$, irregular 

 cavities appear (pc, Fig. 

 135). These are the 

 rudiments of the peri- 

 cardial cavity and they 



