THE BRI'lISH WOODLICE. 



47 



parent, it is necessary for the young creatures to be well 

 supplied with nutritive material. In fact, the bulk of the large 

 egg is made up of food-yolk, on the outside of which the formative 

 protoplasm is disposed in irregular patches. In the fertilized 

 ovum, one of the latter, which lies in a particular position at the 

 end, is found to be larger than the others (see fig. 22). It 

 contains the nucleus of the egg-cell (see fig. 23) and is called the 

 cicatricula. This is the only portion of the egg which divides and 

 produces nucleated cells. It is these which gradually spread all 

 over the surface of the food-yolk, forming a layer known as the 

 blastoderm, which is at first but one cell thick (see figs. 24, 26, 

 and 28). 



Before, however, the food-yolk is quite closed in, a differ- 

 entiation into two layers— the pro-ectoderm and pro-endoderm — 

 takes place (see fig. 25) and rudiments of the first two pairs of 



Cicaizic 

 -ula 



Tluclezzx 

 offfce 



Protoplasm 



Cicalucula 



FIG. 22. — THE FERTILIZED EGG 



(Porcellio sctiber), after roule. 



FIG. 23. — THE FERTILIZED EGG SEEN IN SECTION 



(Porcellio scaber), after roule. 



appendages appear (see fig. 26). Moreover, the cells of the 

 ectoderm change their shape and begin to multiply at two 

 points to form the beginnings of the cerebral ganglia and the 

 nerve cord respectively. 



As the blastoderm closes over the food-yolk, two more 

 appendages arise and these are soon followed by others (see fig. 

 28). A depression appears at the point where "the blastoderm 

 closed and internally the pro-endoderm or inner layer is differ- 

 entiated into two — the endoderm proper and the mesoderm (see fig. 

 29). The former begins to grow so that its edges unite to form 

 the middle part of the intestine (see fig. 29) seen from the outside 

 in fig. 30. The depression already mentioned grows deeper, 

 forming a tube which is the hind portion of the intestine, while at 

 the anterior end of the embryo the front part of the intestine is 

 similarly formed (see fig. 30). By this time also all the nineteen 



