ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 581 



In the ova of the parthenogenetic females the sis chromosomes are 

 found in the prophase of the single maturation-division. No reduction 

 occurs, and the chromosomes divide equally, as in the somatic mitoses. 

 The polar body does not disappear immediately, as in the sexual ova, 

 but remains within the egg near the periphery as a dark compact mass 

 of chromatin, and does not disappear until after the fourth cleavage. 

 There are no perceptible differences in the sexual and parthenogenetic 

 ova at the beginning of the growth period. They originate from the 

 follicular epithelium at the base of the end chamber. 



Cleavage always begins in the centre of the egg. The plane of di- 

 vision for the subsequent divisions varies. Descendants from both 

 daughter-cells of the first cleavage contribute to the formation of the 

 blastoderm. The cleavage nuclei resulting from one of the daughter- 

 nuclei form the germ-band. All the cleavage nuclei do not pass to the 

 periphery in the formation of the blastoderm. Those that remain within 

 the yolk area aid in the digestion of the yolk and prepare it for assimila- 

 tion. The blastoderm begins uniformly over the entire surface of the 

 egg. When the blastoderm is completely formed there is a short inactive 

 period in the sexual embryo. The uninvaginated blastoderm becomes 

 the serosa. The germ-band is completely separated from the un- 

 invaginated blastoderm. The germ-band is of the completely im- 

 mersed type. The parthenogenetic embryo is provided with yolk as 

 needed in the process of development. In the sexual embryo the yolk is 

 completely formed before fertilisation. The sexual males and females 

 develop from parthenogenetically produced ova, while the first partheno- 

 genetic generation develops from sexually-produced ova. 



The primary yolk originates within the cytoplasm of the egg. The 

 secondary yolk originates from the follicular nuclei without the egg. A 

 definite number of parthenogenetic generations are produced before the 

 sexual male and female appear. External conditions do not increase or 

 decrease the number of parthenogenetic generations. The greatest 

 number of winged forms appear in the second generation, especially when 

 food is abundant. The parthenogenetic developing embryo within the 

 winter or sexual egg passes through the winter in a half -grown condition. 

 A distinct male and female line begins in the fifth parthenogenetic 

 generation. The individuals of the presexual or last parthenogenetic 

 generation produce either all males or all females. Only two generations 

 contribute directly to the formation of the male and female, i.e. the fifth 

 and presexual generations. 



Head-glands of Thysanura.* — Jur. Philiptschenko discusses the 

 various kinds of glands which occur in the head of Thysanura. Of 

 special interest are the tubular glands of the last head-segment (the 

 labial segment), which consist of a terminal vesicle and a coiled canal, 

 and excrete through the walls of the vesicle injected ammoniaail car- 

 mine. They are nephridia, which occur also in CoDembola and Diplopoda, 

 though absent in Chilopoda and Insects. Besides these nephridial glands, 

 there are anterior and posterior salivary glands : the posterior glands 

 correspond to those of many insects ; the anterior glands are represented 

 in a few insects. 



* Zeitschr. wiss. Zool., xci. (1908) pp. 93-111 (2 pis. and 2 figs.). 



