160 INTRODUCTION TO CYTOLOGY 



with Menidia Moenkhaus (1904) was able to distinguish easily between 

 the long (2.18 ix) chromosomes of Fundulus and the short (1m) ones of 

 Menidia. Here, as in Crepidula and Cyclops, the paternal and maternal 

 chromosomes form separate groups in the mitotic figure. A similar 

 condition was seen by Tennent (1912) in hybrid echinoderms obtained by 

 crossing in various ways Moira, Toxopnenstes, and Arbacia. In the 

 later cell-divisions the parental chromosomes mingle more or less, but are 

 nevertheless distinguishable. In Fundulus X Ctenolabrus hybrids (Morris 

 1914; Richards 1916), as well as in the normally fertilized Cryptobranchus 

 (Smith 1919), the chromatin contributions of the two parents are dis- 

 tinguishable even in the resting nuclei. 



Size and Shape of Chromosomes.- — One of the most striking evidences 

 favoring the theory of individuality has been found in those plants and 

 animals which show constant differences in size and shape among the 

 various members of each parental chromosome group, so that particular 

 chromosomes are recognizable in the group appearing at each mitosis. 

 Since each parent furnishes a set of chromosomes to the new individual, 

 each kind of chromosome is present in duplicate in the nuclei of this indi- 

 vidual: it is therefore customary to speak of them as being present in 

 pairs, although at most stages of the life history there is ordinarily no 

 actual spatial pairing. 



Since the description of the chromosomes of Brachystola by Sutton in 

 1902 (Fig. 101) the reported cases in which the different pairs of the 

 chromosome complement possess different characteristic sizes and shapes 

 have become increasingly numerous. This is notably true of insect 

 cytology, as is evident in a review of the extensive researches of McClung 

 (1905, 1914, 1917), Robertson (1916), Harman (1915), Carothers (1917), 

 and many others. In the sea urchin, Echinus, Baltzer (1909) found that 

 the 36 chromosomes have constant differences in length and shape, 

 some being hooked and some horseshoe-shaped. In the flatworm, 

 Gyrodactylus, (Gille 1914) there are six pairs, all different in length. In 

 Ambystoma tigrinum Parmenter (1919) finds 14 pairs of graded sizes. 

 In plants may be cited the cases of Crepis virens (Rosenberg 1909; de 

 Smet 1914; M. Nawaschin 1915) (Fig. 56 bis, A), which has three pairs of 

 different size; Vicia faba (Sharp 1914; Sakamura 1915), with five short 

 pairs and one long pair (Fig. 56); and Najas (Tschernoyarow 1914), in 

 which there are seven distinguishable pairs (Fig. 56 bis, B) . In Najas 

 the smallest pair is attached to one of the larger pairs: Sakamura (1920) 

 thinks that these together are really a single pair with pronounced 

 constrictions. 



Not only may certain chromosomes be distinguished on the basis of 

 comparative length, but in some cases there may be other characteristics 

 which serve as marks of identification. In the chromosomes of many 

 plants and animals there are pronounced constrictions in some of the 



