1 48 MCMURRICH. 



EXPLANATION OF PLATE VL 



Fig. 21. Sagittal section through an egg of Jaera showing the mesoderm 

 plug (ine) and the teloblastic cell of the ectoderm plate (Z"). 



Fig. 22. Sagittal section through egg of Jaera showing the beginning immi- 

 gration of the endoderm cells (en) to form the vitellophags. The cells of the 

 mesoderm plug have begun to scatter and the teloblastic growth of the ectoderm 

 has produced short rows of cells (tr). 



Fig. 23. Ventral view of embryo of Jaera in which the teloblastic growth 

 has carried the ectoderm tract over the mesoderm plug. 



Fig. 24. Side view of the same embryo showing the patch of optic cells {E) 

 and the rows of cells extending to it from the ectoderm plate. 



Fig. 25. View of anterior half of an embryo of Jaera in which the yolk is 

 about two-thirds overgrown, showing the scattered arrangement of the cells in the 

 naupliar region and the teloblastic rows in the meta-naupliar. 



Fig. 26. View of posterior half of the same embryo showing the row of telo- 

 blasts (Z") the central one of which (cT) is very well marked. 



Fig. 27. Portion of posterior half of an older embryo somewhat broken by 

 pressure showing the row of teloblasts (T) and the " Mittelstrang " (7ns). The 

 dark transverse bands (nte) in the anterior part of the figure represent mesoderm 

 masses showing through the superjacent ectoderm. 



Fig. 28. Egg of Asellus Communis in the two-celled stage viewed as a trans- 

 parent object. 



Fig. 29. Egg of Asellus in four-celled stage. 



Fig. 30. Egg of Asellus in eight-celled stage. 



Fig. 31. Egg of Asellus passing into the i6-celled stage. 



Fig. 32. Egg of Asellus in the 64-celled stage viewed as an opaque object. 



Fig. 33. Egg of Asellus in the 1 28-celled stage. 



Fig. 34. Blastoderm of Asellus represented as if removed from the ovum. 



Fig. 35. Egg of Asellus at the time of the differentiation of the eleven 

 primary ectodermal teloblasts. 



Fig. 36. Egg of Asellus just after the first division of the ectodermal telo- 

 blasts. 



