344 SUMMARY OF CURKENT RESEARCHES RELATING TO 



chromatin, becoming altered from time to time, or occasionally de- 

 generating, the germ nuclei remain resting. It is tempting to consider 

 that the dense germ-cell determinant is, during this time and later on, 

 acting as a reserve store, while many changes take place in the somatic 

 nuclei. The latter must procure their nutriment from the egg of the 

 host (Do?iacia), and during this they are also becoming disposed ta 

 form the germinal layers. The function of the determinant seems to 

 be that of preventing the germ-cell nuclei from being exposed to the 

 uncertain conditions existing elsewhere in the segmenting egg and 

 embryo. The germ nuclei do not differ at first from the other segmen- 

 tation nuclei. Certain nuclei become germ-cell nuclei only because of 

 their accidental position in the egg, towards the close of segmentation, 

 in the region of the posterior pole. J. A. T. 



Polyembryony in Parasitic Hymenoptera. — J. Bronte Gatenby 

 {Quart. Journ. Micr. Sci., 1919, 63, 175-96, 2 pis.). The important 

 facts are summarized. The poly embryonic Hymenoptera are generally 

 small insects about 1 mm. in length, which lay 1-10 or more eggs in 

 the ovum of the host. The ovum is not killed, but develops into a 

 larva containing the parasite's ova, generally in the hsemocoel. The 

 parasite's egg gives off polar bodies, and may or may not be fertilized. 

 The polar bodies rest for a time, but then break into activity, forming 

 an actively growing mass 6r collection of nuclei. The part of the egg- 

 cytoplasm containing the segmentation-nucleus separates off from the 

 outer part containing the active polar nuclei. The germ-cell deter- 

 minant goes to the former, but later becomes absorbed and lost to sight. 

 The polar cytoplasm or ooplasm containing the polar nuclei forms an 

 investing sheath around the contained embryonic ooplasm which gives 

 rise later to the embryos. The polar ooplasm nourishes the inner 

 embryonic mass, and acts as an amnion (tropho-amnion) or placenta. 

 Its nuclei, derived from the original polar body nuclei, become very 

 numerous. Certain cells of the host-embryo, either h^emolymph or fat 

 cells, or both, form an outer covering to the parasitic germinal mass. 

 This host-covering afterwards becomes much stretched and epithelial 

 in character, but in some forms it is not well developed. The primary 

 embryonic cell which was separated off when the polar nuclei began to 

 be active has now divided many times, giving rise to many germinal 

 masses. This polygerm, lying in its host's haemocoel, goes on forming- 

 more masses, and becomes constricted into areas each containing an 

 embryonic mass, surrounded by two membranes — the outer host- 

 epithelial and the inner tropho-amniotic layer. The shape of the whole 

 may be a ramifying cylindrical body, or an irregular mass constricted 

 here and there by the outer membranes. Each separate germinal mass 

 is now a spherical or ovoid morula containing a score or more cells. 

 The latter keep on dividing. The embryonic or germinal mass now 

 begins to differentiate : the surfaces of the embryo become distinct, the 

 stomodseum and proctodteum are formed by invaginations of the two 

 extreme ends of a ventral groove, the ectoderm is formed by a re- 

 arrangement of the outer cells of the morula, the endoderm and meso- 

 derm are formed in situ by a modification of the more centrally placed 



