page_title: Thomas Harrison Montgomery Jr.
section_title: Cytology
hierarchical_section_title: Thomas Harrison Montgomery Jr. / Career / Cytology
caption_attribution_description: English: Egg development of Lineus gesserensis Fig. 160. Ovum Figs. 161 and 162. Germinal vesicles (as in 159). Fig. 163. Group of neighboring nuclei from a gonad, showing mitotic stages (aq. sol. sublimate&#160;; Del. haematoxyUn, 20 min.&#160;; eosin, 5 min.). Figs. 164-172. Nuclei from gonads (as in 163). Figs. 173-176. Germinal vesicles (as in 163). Fig. 177. The largest ovum found, only a part of the cytoplasm drawn (as in 163).
context_page_description: Thomas Harrison Montgomery Jr. was an American zoologist who made important contributions to cell biology–especially in chromosomes and their roles in sex determination–as well as the biology of birds and several groups invertebrates, naming many species of ribbon worms, rotifers, and spiders. He studied in Berlin before becoming a researcher and professor at the University of Pennsylvania, where he primarily worked until his death at the age of 39. In his short career he published 80 scientific papers and two books.
context_section_description: Harrison published 25 papers on cell biology, primarily using insect cells. His most notable research includes early observations of the pairing of maternal and paternal chromosomes during cell division. He was first to propose that chromosomes play the dominant role in sex determination,  although he rejected the idea that sex was determined by chromosomes alone, and some historians claim he was the first to propose the chromosome theory of inheritance, an idea widely credited to Walter Sutton and Theodor Boveri. He also detailed the morphology of the nucleolus, and observed that in some hemipteran insects the germ cells of males but not females contain odd numbers of chromosomes, which is now known to influence sex-determination, A resolution of the American Society of Zoologists read after his death stated "it would be impossible to write a text-book upon the role of the chromosomes in the determination of sex without referring to his crucial labors in this field."
caption: Montgomery's illustrations of structures in the eggs of Lineus gesserensis, a ribbon worm
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